some people just don't bother learning the ins and outs about self custody, also the learning curve might be steep if we really care about security but honestly, just generating an address and saving it in a paper wallet is easy, if we don't want to care about the ins and outs of security while still trying to self custody we can always use hardware wallet. but despite this, some people just consider learning about a little thing like this to be a pain. even more so for people who aren't really used to technology. so it's understandable.

the thing is, the news about hacker stealing BTC is enough to intimidate many people into thinking that they are just insufficient in term of knowledge to manage their own money, so they decide to just use exchanges's custody.
this self depreciation is the problem I think, where people think self custody have so much thing they need to care about that they feel they just can't do it.