As we all know exchanges can get hacked, caн be down for a long time, can held transactions and so on, so obviously it is not a good idea to keep your coins in them at all times. But then comes the transaction prices, if we move the coins always, we pay extra for the transactions we make. Perhaps the decentralized exchanges where you can directly log in with your wallet will help solve this, as you have the control over your wallet and not the exchanges.
Yes exchanges are the primary target of hackers because they hold huge volume of cryptocurrencies. But if you are trader, the goal is always get a profit so moving your cryptocurrency in and out of the exchange will cost you a lot, so there is no choice but to store it to them until you decided to get your profit. Decentralized exchanges are a good option because it is trustless, transparent and offers anonymity to users but it is not that easy to use and there is no option for traders to trade their cryptocurrencies to fiat and vise versa, this is why decentralized exchanges also have low volumes.