Two extremely obvious reasons:

First, it's a national security issue for a company controlled by the CCP to have intimate data access for hundreds of millions of US citizens. Not only can they glean a great deal of sensitive information, but they have the ability to control the algorithm in ways that benefit the CCP.

Second, China does not reciprocate this level of vulnerability. US companies do not have the same access or control over Chinese users. If you want to allow nation states to diddle around with your citizens, then it ought to be a reciprocal arrangement and then it all averages out.