If you only want to keep your money safe and don't want to use that money much, I suggest you to use different crypto wallets to store your money. But if you want to use your money to do different things or buy or sell different coins then you must use exchange. Most investors don't deposit their money so they keep their money in different exchange wallets. You can also deposit your money in different exchange accounts like them but before depositing you must be sure that the exchange you are depositing money in is safe.
Yeah, many investors are storing their crypto assets on CEXs to participate in saving + staking + IEO, which is not as safe as self-custodying crypto in a personal wallet but it is very popular. Everyone wants to earn income while holding crypto, including stablecoins, and CEXs all have proof of reserves and are quite secure. Binance even has SAFU to be ready to compensate users for losses in many cases.
The only problem is that investors should choose the most reputable CEXs and spread the risk by using multiple CEXs. The collapse of Mt.Gox and FTX is the best warning for all users, after all, CEXs are just a place to trade, not a place to store too many assets.