they would be taking a cut from the staking and if people pool the funds in a 1 entity this makes the exchange be in control of the system. Also this would be a risk if the exchange gets hacked and they would be have a problem in liquidity, people should not trust their money into exchanges.
And for both of those reasons you would be better off staking your own coins. I'm assuming tezos isn't more difficult to stake than any other pos coin, so I'm really not sure why anyone would take this kind of risk *and* profit reduction just to have an exchange be custodian for your coins. Coinbase is regulated by the U.S. gov't, so I don't think the risk of losing money because of a hack is quite as bad as if some unregulated exchange did, but I still don't see why someone would choose to go this route.
i wish Coinbase had an offficial thread here people could discuss things on
