Because most decentralised exchanges are "fake".
Realistically, a decentralised exchange can only operate within a single blockchain, only trading tokens on that blockchain. Works fine for trading between (for example) eth and its various tokens, but you can't do proper eth to bitcoin trades. [1]
The single-blockchain decentralised exchanges are typically real [2], but every multi-blockchain "decentralised" exchange platform cheats.
Some are literally just centralised exchanges that pretend to be decentralised with a slick software interface. The more common technique is to run their own blockchain and have valid decentralised trading on that. But you are only actually trading tokens that represent real coins. They then run a centralised holding entity that holds the real coins and implements deposits and withdrawals just like a decentralised exchange.
There are so many flaws with this scheme.
The holding company could literally run away with your funds. Bugs and exploits (and even normal blockchain behaviour like re0orgs) can result in the three seperate sources of truth (the real blockchains, the holding entity's internal database and the trading blockchain) ending up out of sync.
And just because the trading blockchain is a blockchain doesn't mean the company that controls it doesn't have the keys and power to rollback trades and otherwise change the contents.
[1] Yes, atomic swaps are a thing. But they are resource intensive and slow. I'm not aware of much decentralised trading that uses them.
[2] Though, the topic of wrapped coins complicates things.