This is a mistake that many new investors make so it is important to store your funds in good wallets when buying and selling bitcoin there is a big risk in exchange wallets. If the exchange is hacked or shut down for some reason then there is a possibility of losing your bitcoin this is because the private key of this wallet is with the exchange and not with you that is why you can use hardware wallets where your bitcoin private key is stored safely offline. If you want to hold large amounts of Bitcoin for a long time then you should definitely use a hardware wallet this will give your bitcoin maximum protection.
I think what these exchanges are good for is trading and other quick transactions but not for storage of funds especially if these funds are not what you intend to use for your daily needs. Mostly what I think many people leave in their exchanges are little funds for expending but anything outside such funds should be kept in the cold wallet which serves as your storage and do not allow carelessness or laziness to go and store all your assets on yur exchange to avoid stories which breaks the heart.