The exchanges especially decentralized exchanges can't be trusted because they can be rug-pull schemes. I have seen many decentralized exchanges that scammed their users and took a rug-pull type of exit. That's another reason to not trust the decentralized exchanges.
Then they weren't truly decentralized. If you're referring to the well-known DeFi, 99.9% of it is not decentralized; it is not even exchange. It is just a manner to use blockchain, so you can call your activity "decentralized", where it's just you who makes the rules.
If you make use of a truly decentralized exchange like Bisq, then you'll see it can be trusted. It runs for years and is as decentralized as it gets with the operation of a DAO:
https://bisq.wiki/Main_Page.