the reasons everyone still keeps their crypto assets on exchanges will vary widely.
and these reasons certainly have a diverse rationale. like some because they are really beginners and don't even know about the risks they take when they leave their assets on the exchange.
and of course there are those who only keep part of their assets on the Exchange. the reason is because they trade it every day (a day trader). and there are also reasons because they only have small assets so they don't have to bother transferring them to their personal wallet.

and i think a beginner who come with a lot of money (rich) and he will start to invest in crypto and bitcoin. and he will invest heavily. so I think the beginner will definitely first explore a safe way to store his investment assets.
there's no way he's going to deposit a lot of money and buy a lot of bitcoins on the exchange and leave it be. I think for now even beginners already know where they have to store their assets to be safe. but most of them still keep some of their crypto assets on the exchange because they really continue to trade them or indeed the amount is really, really small.