Crypto is indeed a decentralized system (well, the well-functioning ones at least).

However, it's also a closed system and you need to get crypto from somewhere. A centralized exchange is the most efficient facilitator of buying/selling crypto, and after that you can withdraw it and enjoy digital transfers without a centralized party.

(You can also get crypto buy mining or selling things for it, or even buy it for cash, but an exchange like Coinbase is much faster and more convenient.)