1. Markets are aware of that and lend to China accordingly (the ones who are free to choose who to lend to).

2. Do you think Americans would tolerate having a government that purposefully suppresses their purchasing power through structurally low wages, and buying up foreign assets to supress the value of their own currency?

3. The part where China spends all their cash to buy US Treasuries to supress the value of their currency wouldn't really work as well for the USA when international trade is dollar denominated anyways, and the bulk of their GDP comes from domestic consumption.

4. The USA has a deficit. It'd need to borrow even more in order to finance the suppression of their currency, whereas China just redirects the money they earn from export surpluses.