Might sound dumb, but there are still places in the world where having $400k worth of BTC != $400k in the bank.
Plenty of banks will freeze your bank account instantly.
And good luck convincing them that you stumbled upon $400k by solving a puzzle - only takes one suspicious fraud/risk manager to conclude that there’s a higher chance of fraud than legitimacy.
(But you are right, no problem to find someone to pay you the market price. That’s a done deal in seconds)
Examples of such places? I did KYC/AML with multiple European banks when withdrawing crypto, and while they checked thoroughly, there were never any issues.
https://news.ycombinator.com/newsguidelines.html
"Be kind. Don't be snarky. Converse curiously; don't cross-examine. Edit out swipes."