It would take only a single e-mail and the U.S. could go after the non-custodial P2P platforms that have no KYC/identity verification requirements (i.e., HodlHodl, Bisq, LocalCoinSwap) by seizing the domains. But they haven't.

They could go after the traders using those platforms with sting operations. But they aren't (as far as we know, albeit with a couple known exceptions in a few countries, such as Nigeria).

They could. But they aren't.

If I had to guess it is because they can monitor (orders are public, and they can enter trades to obtain the counterparty's identity from the fiat payments).

Might they go after these platforms or traders in the future? I suppose so. But that would push it further below their radar, which is probably not something they want to see. And they got a lot bigger fish to fry than the plebs buying and selling a little bitcoin here and there, without being KYC required.