they are faster and cheaper, but they are not kyc free, and a lot of people don't have access to CEXs for various reasons. so exchanges like eXch are their only option.
I would argue if those big centralized exchanges are always cheaper and faster.
This is the case in the vast majority of the cases.
Just remember that the added liquidity is the price of your privacy,
It's not that DEXs are dearer; the centralised exchanges offer discounted spreads because they thrive on selling your data to very nosy actors.