They even noticed it before the FTX crash, but they believed in the exchange so much they didn't mind it. However, as far as FTX is concerned, there are many negative issues that may make them uncomfortable keeping their assets on the exchange so they choose to withdraw their assets to their respective wallets.

But as @mk4 says, these assets will be very easy to transfer back to the exchange when their holders start to trust the exchange again as before. For a while, the situation of centralized exchanges really lost a lot of holders as most of them withdrew their assets, but it will return to normal with time.