DEX's are so needed in the crypto sphere that I can't believe there isn't a major one operating yet. To realize the decentralized, uncensorable founding principle of crypto, a DEX is the logical next step and is needed to achieve that vision
they alreayd exist, you have etherdelta, waves. but they are still on beta, working on their product, the centralized one keeps the market for the moment, but soon the liquidity will flow on those platform.