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.
 The history of Tibet ruled by ROC, 1912~1951, wikipedia
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.