But why? Why would you do that?

Its pretty simple. Crypto is very secure, but crypto exchanges are like banks and can be scammed and robbed by employees. Many exchanges got robbed by employees.

Bitcoin is based on SOMETHING. Sure it's only based on hours of computing power, but it's based on SOMETHING.
FTX's own crypto, as well as most crypto derivatives are based on nothing. They were selling the right to gamble on the right to gamble on bitcoin's value. And of course the people investing in these fell for it because they viewed it as a "stock." There's a sucker born every minute.
And no, crypto is not secure. World governments or banks can just make an arbitrary decision and cancel crypto, just like any digital currency.

It's all based on how much electrify is used to mine it and confidence in the currency, except crypto like ether. Ether has code in it which is worth money. They call them smart contacts but essentially they are programs that live in the blockchain.
They have intrinsic value, just like any other software.