Many answers, the most immediate being that politicians doodling Chinghai shapes on paper in Shanghai doesn’t represent functional annexation.
Case in point, Mao’s declaration, upon liberating Lanzhou, that China in its entirety had been liberated, the PRC formally established. Not one mention of Tibet on that day.
Mao himself declared the functional borders of Tibet on October 1, 1949.
He did so by declaring, on that date, that the whole of the People’s Republic of China was formally and completely established. No Tibetan regions were represented, yet as he stated in his address, “all regions of the PRC are represented.”
On 2 January 1950, Mao telegraphed Peng from Moscow to reiterate his determination to invade Tibet: “The population in Tibet is not large, but Tibet has a very important international position, and we therefore must occupy it and transform it into a Tibet of people’s democracy."
The day before, Mao had met with Stalin and they had the following exchange:
Mao Zedong: I would like to note that the air regiment that you sent to China was very helpful. They transported 10 thousand people. Let me thank you, comrade Stalin, for the help and ask you to allow it to stay a little longer, so it could help transport provisions to [CCP CC member and commander of the PLA’s Second Field Army] Liu Bocheng’s troops, currently preparing for an attack on Tibet.
Stalin: It’s good that you are preparing to attack. The Tibetans need to be subdued. As for the air regiment, we shall talk this over with the military personnel and give you an answer.
(Minutes of Conversation between I.V. Stalin and Mao Zedung, Moscow, Marxist Internet Archive)
Of course many hundreds of years before all this, Tibet itself annexed vast swaths of China. In my opinion, both China and Tibet have possibly the most liberal interpretation of “border” in human history, lol. The “border” itself, as claimed by both nations across history, could constitute by landmass alone a third country. China hadn’t “annexed” any more of Tibet than Tibet had of China–less, in fact, if you use linguistic and architectural data to establish who had the most control over this area throughout history.
That a hybrid population was not established, in fact, is a testament to the significant differences–linguistic, cultural, ethnic, and social–between the two nations. Oil and water.