Because that's the main use. Transactions through bank accounts.
Assume that banks and governments don't know that you have trades related to cryptocurrencies but if you are trading with a trade partner that is identified by banks and governments, they will find you and ask for more details, evidence of your bank transfers. It's when your trouble starts to appear and you can imagine what might possibly happen next.
Exactly, this is how it really happens. It doesn't come directly from the DEX, but from when one of the trader accounts is being suspected and confirmed to be doing a cryptocurrency-related transaction, then they will trace all the people they did transactions with. Questions will be asked, and the purpose of the transaction will also be asked, which, if no concrete evidence to differentiate you from the accusation, the next person and every other person they have done transactions with will be implicated. What's needed is for one person to be suspected, and the rest will be affected.
Technically, yes.
I believe the best thing to do to limit the chance of being questioned in a situation like this is to never do crypto to fiat trading twice with a single person, just like it was advised not to use a single crypto wallet address twice.
Having said that, I look deep into the move of the EU government concern the ban of anonymous crypto and token in 2027 as a means to make institutional banks as the major player for crypto to fiat trading in the future, and I hope every crypto holder will prioritize noncustodial holding more than before.