Because it would drop to a zero dollar value before you could ever cash even a fraction of it out or buy anything, as soon as there's clear evidence on chain that someone actually stole crypto by simply using brute force, which could only be done with a quantum computer under very specific circumstances and limited time frames.

Consider it a protection mechanism, it is fully possible to reverse a blockchain into a certain state, say moments before it was breached, after updating the encryption algorithm so that it won't happen again. All it takes is that a majority agrees on it and you'll end up with a fork. You now have Bitcoin (old and hacked) and Bitcoin (current and secured).

Most other applications relying on encryption won't recover that easy.