Holding millions of dollars worth of coins yourself, rather than in a bank can become quite nerve wracking. Open your wallet on the wrong computer, or let someone untrustworthy to see where/how you hide your key, or how much you own, and you can wake up one day with all your funds gone.
I know there is the other side of that, that the government can seize your funds in a bank account, but realistically they won't target you out of millions of people who have millions in their name.
That's what hardware wallets, paper wallets, and safe deposit boxes are for. It doesn't take that much effort or technical skill to keep coins very secure, and the information on doing it well is readily available.
At some point you have enough money that it doesn't really matter. If you had $20k in Bitcoin you may be tempted just to keep holding. If you had $3mm, who really cares? Sell half or two thirds to diversify if you still believe it has room to grow or just drop it all into more traditional investments and take your interest and dividends for the rest of your life.
(Not that I'm arguing or worrying. I was just curious.)