the reason for your question is because the centralization exchange (cex) first appeared before the Decentralization Exchange (dex), if if first the DEX appeared first I am sure many people would prefer DEX over CEX now,
if DEX is already many and famous then I'm sure many people will switch to DEX later.
It's all a matter of time you know when Decentralized Exchanges take over Centralized Exchanges. The reasons are pretty simple you know, the fees that are being charged by most CEX are way too high. Also, you have to submit proof of identity and all of that but with DEX you don't do any of these yet you get to make your transactions smooth and swift.