Many people will never be capable of self-custody because the lack the interest to do it. They see money to be made and ignore the warnings of those who try to explain what they have on an exchange is a promise of money, not money itself.
When you say "nobody does this anymore," that's kind of true, but also not true. Those who have learned the hard way do, the newbies (which vastly outnumber the first group) don't.
Self custody requires knowledge of some basic math, cryptography, and the ability to understand basic security principles.
User growth explodes with exchange rates. Those diving in understand very little about what they're doing and should stay out. They don't listen to people saying such things and the result is, well, predictable.