Neither we should trust DEX smart contracts to store them for too long time. As at least when we trust CEXes, they get sued after messing up, You can't sue a faulty hacked smart contract. And what comes to praising decentralization in general, it almost always has centralized elements. Like wrapped btc that just got depegged supposedly because of
Almada Research minted it a lot.
DEX also cannot be fully trusted, because some DEXs are managed by scammers who will easily take our assets. As you said, no one can sue Smart Contracts if they commit fraud and take the assets of the users, they will just disappear. In contrast to CEX which will be demanded by its users directly. The main security is actually in its own use. they must be aware that crypto is very risky, and must be really good at keeping their own assets safe. A personal wallet is a safe place that can be managed alone.
No matter how you look at it, exchanges are not meant for your assets to be stored with, for a long period of time. For reasons even if the FTX incident happened; people would get easier access with your funds especially if you are using mobile phone. Unlikely with wallets wherein you'll be needing passcodes from time to time you are accessing your funds which means better security.Another thing is being easily tempted with making your action such as selling or buying. Decisions should be well backed with ideas before engaging into it. If your funds will be stored in wallets, you'd be more hesitant 'moving'. These are just more practical reasons why putting your assets in a wallet, would be much better. Practical and logical reasons of why it should be done is better than to think of reasons because of the recent issue, which may just feed the worries of new ones in this industry.