Keeping your Bitcoin in an exchange is really an unwise thing to do because if anything happens to that exchange in question, like hacking or internal theft, or like in the case of FTX, just know that you asset is gone, that's why it's advice to store your Bitcoin in a self custodian wallet like electrum where only you have access to
FTX is very big black swan event from a centralized exchange but you can lose all your money without a big case like FTX. Fortunately, FTX users started to get compensations from the exchange but in most cases of hacked exchanges, scam exit ones, bankrupted ones, you have to assume you will lose most or all your money.
It's very unrealistic to think that after a blackswan event, you will get full amount of your fund or most of it back.
and one funny thing is that, most investors have heard of this statement before which is not your key, Not your coin, but they really don't understand what that statement mean
They can understand it but they did not mind to practice properly according to security advice. They naively thougth that they will be well, and practiced carelessly until they lost money but at the accidental time, it's too late as Bitcoin transactions are irreversible. What lost is lost forever and can not be reversed.