Centralized exchanges do provide wallets for users to store their cryptocurrency. However this is the problem. Because it is a centralized exchange, they use centralized servers which can be a target for hackers, and also they have a central authority who can ultimately manipulate the exchange for their benefit.
Decentralized exchanges do not provide storage for your cryptos, you are responsible for maintaining a secure place for your cryptocurrencies. They ensure a safer environment for you to make trades and they remove the third party between you and the person you are trading with.