Since we ruled corporations are people too in the Supreme court, could the major media companies sue Trump or the US government for damages in regards to defamation?
This would probably fit better on Law.SE, but we did not rule that corporations are people. The Supreme Court ruled that when people assemble into a corporate form, they do not lose their free speech and press rights. They also don't lose protections against slander and libel, but that wasn't established in the more recent decision.
The US government is not making any statements, just one member of the US government. Admittedly, the most individually powerful member of the US government, but just one. And especially since that member is chosen not by "the government" but by the people, it's not the government's responsibility that a good one be chosen.
They could sue Donald Trump personally. However, there are some challenges to this:
The relevant entities would probably be covered under the public spotlight rules. It's much harder for the publicly well known to sue for slander or libel because the standards are higher.
The underlying sentiment is hard to characterize as incorrect: fake news is the enemy of the American people.
It's trivial to establish that important members of those organizations believe that they have a responsibility to weaken Trump. I.e. that they are actively looking for anti-Trump material to publish. Why trivial? They've said it, on air or in print. There are probably other private statements that could be brought out in discovery.
Truth is a defense against slander/libel. So if those same entities have run any stories that were fake, particularly if they can be shown to have done so knowingly, then Trump would be protected in his opinion on it.
Each of those entities has run stories that they either knew were fake or that they should have known. Some of that news about Trump but also about others.
Just off the top of my head:
ABC had the Food Lion case, where they filmed an ABC employee shelving expired food; another ABC employee putting the food item in a shopping cart; and a third ABC employee ringing up the food as the cashier. Food Lion won a multi-million judgment from the jury which was reduced to hundreds of thousands by the judge and to $2 by the appellate court.
NBC had the exploding pickup case, where they put an incendiary device on a pickup so as to make it explode on camera.
CBS used fake documents in a story about George W. Bush. Both the alleged author and forensic experts have said that the documents are not accurate.
Pretty much every news outlet played an edited version of George Zimmerman's 911 call that made him look more racist than his actual conversation supported. For example, by editing out "Or he’s on drugs or something. It’s raining and he’s just walking around, looking about." and "Was he black, white, or Hispanic?" which juxtaposed "This guy looks like he’s up to no good." and "He looks black."
The news media often runs stories that they haven't fully vetted. This leaves them vulnerable to charges like "fake news." Of course, the success of that accusation can then lead people to use it in regards to real news.
In order to win a lawsuit over a fake news accusation, they would have to prove that that specific news was not fake. That Trump knew that it was not fake and maliciously chose to call it fake anyway. And that they suffered damage as a result. It's not an easy standard to meet. Note the Food Lion case. They couldn't meet the defamation standard and eventually only won on trespassing.
I'd say that I suspect that Trump could afford the $10 for the five organizations that he called fake news, but he didn't trespass. By the Food Lion standard, he's clear.
I mean, we are talking about the President of the United States blatantly saying the majority of the US's entire news media infrastructure is "fake news" - this is unprecedented.
Those five organizations aren't the majority of the US's entire news media structure. The New York Times is one of three elite newspapers (along with the Washington Post and the Wall Street Journal) but there are many smaller papers (including several in New York City). ABC, CBS, and NBC are the bulk of the broadcast network news, but there are other smaller broadcast news organizations. CNN is one of four cable news networks, albeit the oldest of them.
And as to unprecedented, Barack Obama made the same kind of allegations ("not a network") against the most popular of the broadcast cable news networks. Perhaps only one, rather than five, but certainly not unprecedented. That word is overused.
I suspect that if you went back farther, you could find more precedents. It used to be more normal for various outlets to have known biases. In 1860, there were Republican newspapers and Democratic newspapers.