China buys US debt so it can peg the Yuan to the dollar to keep volatility low and manage its export levels accordingly. China only owns 5% of outstanding US national debt, so even if they sold it off in a flash sale for some reason, someone else would buy it (other nations or more likely the Federal Reserve). Selling off US debt is not a credible economic threat for China. [1]
No powerful US entities have a vested interest in keeping foreign Chinese money out of US real estate, especially if it's due to wealthy individuals investing abroad. The real estate owners win, the developers win, construction companies win, the mortgage-underwriting banks win, the losers are the young/renters/non-capital owners seeking jobs in these hotspot areas because their rent just goes up and up per square meter. This segment of the population has no political power, they can only hope to make enough to put a down payment on a house and join the vested interests as capital owners - hopefully the bubble doesn't burst taking their job and leaving their mortgage underwater. The exception here would be if a Chinese government-owned investment vehicle tried to purchase large swathes of real estate, it could potentially use this large investment as a weapon to control cost of living/tank real estate values in that area if the government wanted to.
The real reason is because US technology (networking, telecom, A&D, automotive) companies have a huge vested interest in keeping their competitive advantage over Chinese technology companies which are on the cusp of transforming from cheaper/lower quality alternatives to preferred suppliers in building out information technology infrastructure globally, including in the US. The US government also has a huge vested interest in controlling this infrastructure, especially in the US, or at least being on good terms with the companies that build and manage it. It doesn't want partially government-owned and directed companies like Huawei supplying complicated and sometimes unvettable hardware and software over which Americans send each other vital information.
[1] China's central bank uses a modified version of a traditional fixed exchange rate. That's different from the floating exchange rate the United States and many other countries use. The People's Bank of China manages the yuan's value. It keeps it fixed to a basket of currencies reflecting its trading partners. The basket is weighted toward the dollar since the United States is China's largest trading partner. It keeps the yuan's value within a 2 percent range against that currency basket.
Why does China fix the yuan's value to the dollar? It manages its currency to control the prices of its exports. It wants to make sure its exports are reasonably priced when sold in the United States. Every country would like to do this, but few have China's ability to manage it so well. China's command economy allows it to control the central bank and many businesses. As a result, the Communist Party directs China's economy. The U.S. government regulates exchange rates instead of managing them.
https://www.thebalance.com/how-does-china-influence-the-u-s-...