Reports in the media (that I consume) appear to show China's continuing actions to exert control over Tibet. What does China have to gain by controlling a population that it appears will have little bearing on China's prosperity, and will actively alienate its populace?
Two of the longest rivers in China originate in Tibetan Plateau. China could face very dangerous threats without Tibet under its control:
Several major rivers have their source in the Tibetan Plateau (mostly in present-day Qinghai Province). These include the Yangtze (longest river in Asia, Yellow River (third longest river in Asia, Indus River, Mekong, Ganges, Salween and the Yarlung Tsangpo River (Brahmaputra River)
Tibet has huge reserves of copper, lithium, gold and silver. Most of it has never been touched, because the Tibetans didn't mine the land: it's against their religious practices to disturb the ground. But China has begun mining on an enormous scale.
Its location is strategically important as it's bordering India, China's rival.
As @DylanChensky points out in the comment below, China has some conflicts with other minority ethnic groups, especially in the West. If China loses control over Tibet, other ethnic groups might take the opportunity to liberate themselves from China's control.
Considering all the factors above, China will never lose control over Tibet.
This answer is primarily a large comment that expands on point 4 of Rathony's answer, which I'll paraphrase as "not wanting to set a precedent". The official party line is that Tibet was part of China since 1271, continuously. Because that's what the Chinese populace is taught, government leaders need to assume Tibetan independence/sovereignty would lead to separatist movements in other independence seeking provinces increasing their efforts.
The status of Tibet before 1950, especially in the period between 1912 and 1950, is largely in dispute between supporters and opponents of Tibetan independence.
According to supporters of Tibetan independence, Tibet was a distinct nation and state independent between the fall of the Mongol Empire in 1368 and subjugation by the Qing Dynasty in 1720; and again between the fall of the Qing Dynasty in 1912 and its incorporation into the PRC in 1951. Moreover, even during the periods of nominal subjugation to the Yuan and Qing, Tibet was largely self-governing. As such, the Central Tibetan Administration (CTA) views current PRC rule in Tibet as illegitimate, motivated solely by the natural resources and strategic value of Tibet, and in violation of both Tibet's historical status as an independent country and the right of the Tibetan people to self-determination. It also points to PRC's autocratic and divide-and-rule policies, and assimilationist policies, regarding those as an example of imperialism bent on destroying Tibet's distinct ethnic makeup, culture, and identity, thereby cementing it as an indivisible part of China. After the fall of the Qing Dynasty, both Mongolia and Tibet declared independence and recognized each other as such.
On the other hand, opponents assert that the PRC rules Tibet legitimately, by saying that Tibet has been part of Chinese history since the 7th century as the Tibetan Empire had close interactions with the Chinese dynasties through royal marriage. In addition to the de facto power that the Chinese has since then, Yuan Dynasty conquest in the 13th century and that all subsequent Chinese governments (Ming Dynasty, Qing Dynasty, Republic of China, and People's Republic of China) have been exercising de jure sovereignty power over Tibet.
In addition, as this position argues that no country gave Tibet diplomatic recognition between 1912 and 1950, they say that China, under the Republic of China government, continued to maintain sovereignty over the region, and the leaders of Tibet themselves acknowledged Chinese sovereignty by sending delegates to the following: the Drafting Committee for a new constitution of the Republic of China in 1925, the National Assembly of the Republic of China in 1931, the fourth National Congress of the Kuomintang in 1931, a National Assembly for drafting a new Chinese constitution in 1946, and finally to another National Assembly for drafting a new Chinese constitution in 1948. Finally, some within the PRC considers all movements aimed at ending Chinese sovereignty in Tibet, starting with British attempts in the late 19th and early 20th centuries, to the CTA today, as one long campaign abetted by malicious Western imperialism aimed at destroying Chinese integrity and sovereignty, thereby weakening China's position in the world. The PRC also points to what it calls the autocratic and theocratic policies of the government of Tibet before 1959, as well as its renunciation of South Tibet, claimed by China as a part of historical Tibet occupied by India, as well as the Dalai Lama's association with India, and as such claims the CTA has no moral legitimacy to govern Tibet.
Kublai, the King of Chinese Yuan Empire will answer:
"Because I like it".
---- Just kidding, but:
The fact is Tibet was conquered and became part of China since 1271, and this situation has never been changed for 700yrs.
Tibet become part of China (conquered by Yuan Empire) since 1271, and after the rest of 700~yrs, though Tibet is invaded by British for several times, there's never ever a single minute that Tibet become independent district or a colony.
See, it's totally different from once a part of China, it's lasts as part of China for 700 yrs.
That's why we say what Chinese government is doing today, it is just protecting its inherent territory.
------ Reply one comment here because this is important -------
"If having held territory once in the past were sufficient reason to reclaim it, the EU would be a nuclear wasteland by now"
Sorry, but wrong. If you say EU, you should point out when Tibet became independent again after 1271 and when China begin to reclaim it.
Or, define reclaim please.
------ Updated answer -------
I believe the disputed part should be 1912-1951, during which time Tibet is ruled by Republic of China (inherited from Qing Empire), some people call it "defacto independent".
But, this defacto independent is totally made up, and never be recognized by any country
The truth is, this independent is just a farce conducted by British intruders and never be admitted by China, US and other countries (and none of them signed any agreement about Tibet).
They may say Tibet even had its own "passport". But again, the truth is, that "passport" has never be admitted by other countries :
In this regard, the US Embassy staff replied that the United States has always recognized China's sovereignty in Tibet, the US government has not changed the meaning of the position of Tibet. US State Department staff also replied to the Chinese ambassador to the United States Gu Weijun said: the US Consul General in Hong Kong did not issue visa on this passport, instead they only signed in another plain paper that they are allowed into the United States border, the "visa" means no damage to China 's Sovereignty over Tibet.
See, this is how US Embassy reply to ROC's inquiry about Tsepon Shakabpa's "Tibet passport"
You may also noticed the English version of Tibet history seem like telling a different story: https://en.wikipedia.org/wiki/History_of_Tibet
Luckily you can find the missing part from reference , for example, how US Embassy reply to "Tibet passport" which was claimed as the strong (but only) evidence that Tibet was a independent country.
Another example I'd like to mention is HongKong, that's reclaim. Because Chinese gov lost the war and signed the agreement, and then reclaimed it 99yrs later. But Tibet? Not this case.