It depends by what you mean by "acts alone". You seem rather unaware of the level of moral support she and the broader school-strikes-for-climate movement has received:
On 31 January 2019, more than 3400 scientists and academics signed an open letter in support of the school strikes in Belgium. The letter reads "On the basis of the facts supplied by climate science, the campaigners are right. That is why we, as scientists, support them." This was followed by an open letter in support of the school strikes in the Netherlands, signed by 340 scientists, and by 1200 researchers in Finland signing a letter, on 11 March 2019, supporting the strikes. An article published in Nature in March 2019 listed many other expressions of support, and no criticisms, from scientists, with comments such as "The idea of a climate strike is innovative. It’s provocative, and I think it’s the right form of non-violent civil disobedience".
In Germany, Austria, and Switzerland a group of scientists founded Scientists for Future (S4F) in support of the factual correctness of the claims formulated by the movement. The statement was signed by over 26000 German-language scientists and scholars.
On 14 March 2019, the Club of Rome issued an official statement in support of Thunberg and the strikes, urging governments across the world to respond to this call for action and cut global carbon emissions.
In early April 2019, a letter titled "Concerns of young protesters are justified" was published in Science. The letter declared that the climate strikers' concerns are "justified and supported by the best available science" and was signed by over 3000 scientists worldwide.
In June 2019, 1000 healthcare professionals in the UK and elsewhere, including professors, eminent public health figures, and former presidents of royal colleges, called for widespread non-violent civil disobedience in response to "woefully inadequate" government policies on the unfolding ecological emergency. They called on politicians and the news media to face the facts of the unfolding ecological emergency and take action. They supported the school strike movement and Extinction Rebellion.
If I am to speculate (and be a little cynical), Greta becoming a center figure is fairly easily explained by a number of factors:
Lack of any other international "eco warrior" figure of a comparable age. Sure, politicians like Al Gore have championed action on climate change before (he even got a Nobel Prize in 2007 for that), but can you name someone else in Greta's age bracket? (Sure a few other names became recognizable in the aftermath of the school strikes movement, but there is that "founder effect".)
She speaks good English, which gives her access to an international audience. She is also quite articulate in her speeches.
Her determination and steadfastness. It results in interesting enough news pieces like on her travel choices, her influence on her family, e.g.
Unlike most global figures, Thunberg doesn’t have a staff; her parents do what they can to maintain a sense of normalcy for her and her 13-year-old sister, Beata, though Svante no longer answers the phone unless it’s a trusted contact.
Meantime, there is a Greta effect within the home too. Svante and Thunberg’s mother Malena Ernman have given up meat, installed solar panels on their home and stopped traveling by air—decisions they made because they tired of arguments with their stubborn daughter, Svante likes to joke. It’s been a major shift for Malena, an opera singer who no longer flies abroad to performances. “Once she realized the consequences of that lifestyle, she was easy to convince,” Thunberg says, sounding more like a parent than a child.
There is a level of fascination with her in the media because of her atypical personality. Even when she was attacked for that (by detractors), it created even more media coverage when the detractors were criticized etc.
Success breeds success. Her UN speech was hardly the first in front of an audience like that. She also spoke in front of some national assemblies (at least the French one and the UK one) and the EU Parliament.
Her family is reasonably well-off. Her father (a former actor and now a writer) can travel with her, etc.
Accusations that she is paid have been levelled before,
Since she came to prominence, Thunberg has been the target of negativity, trolling and even threats. Right-wing commentators and climate-change deniers have called her a “PR puppet” who is paid by a global network of billionaires to spread a “left-liberal” message.
But insofar these have been proven nothing more than a canard. The wider school strike movement was also the subject of similarly unsubstantiated claims:
A Belgian environment minister has been forced to resign after falsely claiming the country’s intelligence services held evidence that children skipping school to demonstrate over climate change were being directed by unnamed powers.
Joke Schauvliege, a minister in Flanders, where the school-strikes movement first emerged, provoked a wave of criticism of the wider political class after suggesting the protests were a “set-up” and “more than spontaneous actions of solidarity”.
“I know who is behind this movement, both of the Sunday demonstrations and the truants,” Schauvliege had told an audience of farmers. “I have also been told that from state security. I can guarantee that I do not see ghosts alone and that climate demonstrations are more than spontaneous actions of solidarity with our climate.”
The comments prompted a rare denial by the Belgian state security services. “We have not reported anything about this to Schauvliege,” they said. “Neither verbally nor in writing.”
Schauvliege resigned in the aftermath.
There is a less palatable aspect to Greta's rise, which does involve a businessman. As she writes:
On August 20, 2018 I sat down outside the Swedish Parliament. I handed out fliers with a long list of facts about the climate crisis and explanations on why I was striking. The first thing I did was to post on Twitter and Instagram what I was doing and it soon went viral. Then journalists and newspapers started to come.
A Swedish entrepreneur and business man active in the climate movement, Ingmar Rentzhog, was among the first to arrive. He spoke with me and took pictures that he posted on Facebook. That was the first time I had ever met or spoken with him. I had not communicated or encountered with him ever before.
Many people love to spread rumors saying that I have people “behind me” or that I’m being “paid” or “used” to do what I’m doing. But there is no one “behind” me except for myself. My parents were as far from climate activists as possible before I made them aware of the situation.
I am not part of any organization. I sometimes support and cooperate with several NGOs that work with the climate and environment. But I am absolutely independent and I only represent myself. And I do what I do completely for free, I have not received any money or any promise of future payments in any form at all. And nor has anyone linked to me or my family done so.
Apparently Rentzhog was looking to profit from Greta's fame. Wikipedia says:
In late 2018, Ingmar Rentzhog, who claims to be one of the first to publicize Thunberg's climate strike, asked her to become an unpaid youth advisor to his climate startup company. He then used her name and image without her knowledge or permission to raise millions for a WDHT for-profit subsidiary, We Don't Have Time AB, of which Rentzhog is the chief executive officer. Thunberg received no money from the company. She terminated her volunteer advisor role with WDHT once she realised they were making money from her name, stating "[I am] not part of any organization… am absolutely independent… [and] do what I do completely for free."
Rentzhog has been unapologetic about his approach.
Finally, Thunberg's family did write a book about their experiences with the sudden fame, but apparently all money from that is going to charities; Greta writes
Before the book was released my parents made it clear that their possible profits from the book ”Scener ur hjärtat” will be going to 8 different charities working with environment, children with diagnoses and animal rights.