You may have heard of James Howells and his $11 million plan to find a hard drive with $181 million in Bitcoin. That is a risk of self-custody. His chance of finding his bitcoin is actually not too small.
If you had bitcoin held on FTX, there is no amount of money or time that will bring those coins back. They are gone. You have a 0% chance of finding those bitcoin.
Not your keys, not your bitcoin.
The subject of this thread is that self-custody risks outweigh the risk of exchanges, and they you seem to provide an argument in direct contrast with that subject line. Anyway, there are definitely risks to self-custody, but stupidity isn't one you should be too worried about, unless you're an idiot that might throw away your private keys, in which case it might be better to store them on exchanges. I think a bigger risk is the $5 wrench attack or having your computer infected with some sort of virus. This is why people give ways to secure BTC without needing to be connected online and you should take measures to make sure a wrench won't be able to expose your private keys (not having direct access to them is a win).