It's not just them.

When you buy BTC on a Bitcoin exchange, you're not buying BTC. You're buying a ticket to get BTC, if you choose to withdraw it.

Only when you choose to withdraw it (and control the private keys) does the Bitcoin exchange actually have to send you any BTC.

So your online balance from the Bitcoin exchange is entirely and completely meaningless.

All Bitcoin is paper Bitcoin until it's self custodied.

The only thing that matters is BTC where you control the keys. Everything short of that is bullshit paper BTC.

Self custody is a must with BTC. Everything short of self custody is just free money for rich assholes to steal from you eventually.