I'll agree that crypto self-custody isn't really something the vast majority of folks can safely do. That said, self custody of crypto is different than self custody of fiat currencies.

With fiat currency, you have to physically store the cash in 1 location. With crypto I can store multiple keys to my crypto on physical devices that require a pin to unlock (and reset after a few failed attempts). I can store my key across multiple locations in such a way that n of m copies of the key are required to move funds. I can require a waiting period before moving funds. I can require another person to approve the transactions in certain scenarios (ex: moving large amounts of funds).

I mean, it is - you can put your cash under your mattress. But there is a reason you don't.