Alot of newbies make this mistake of considering exchanges to be their wallet that they trust them to store their crypto coins for them but that's a big mistake that alot of them end up making. Exchanges aren't wallet but a place to only buy and sell and that's what we should be using it for. Anyone using it as a wallet or for any other purposes that they weren't created for are taking a big risk.
They do neither know about fundamentals and importance of wallet mnemonic seed and private key nor risk of losing all their funds by relying on other parties in owning their wallet private keys. The third parties can be scammers, or can be hacked so private keys of customers' wallets can be compromised and used to steal customer fund on exchanges.
Many hacks happened in history of Bitcoin and cryptocurrency market.
Many exchanges died after hacks or used hacks as a reason for their scam exits.
Not your keys, not your coins.Cryptocurrency exchange graveyard.List of Major Bitcoin Heists, Thefts, Hacks, Scams, and Losses