When you first start crypto and even in the forum now under the beginner section you are told to never leave your coins in exchange and to finish your business and withdraw. Staking on an exchange goes against this advice. When you stake you are buying a significant amount of coins and risking losing them all. Why not just stake them yourself? Why pay an exchange a cut and give them control over your coins?

If you are talking about nodes then that is another thing. This is not staking, it is similar though. The reason people share nodes is due to the high price of some of them. With nodes, I can understand using a third party and would recommend an exchange over a "node staking pool" ultimately though you want control of your own investment.