Because the governing agencies that created that fiat will not allow simply anyone to create an exchange. China and USA are already two huge examples. The exchange and the blockchain must comply with the rules of the government their customers live in. (AML) and (KYC) are two popular ones that prevent decentralized exchanges opening in certain countries.
MINDS.com uses points instead of currency. Their points are not marked as a security by the SEC, as they are not traded openly on any market and therefore do not need to follow the same rules.

This argument started over 10 years ago when the USA made it illegal to transfer funds from a bank account to a known poker/gambling website. So the website owners got smart. Instead of using cash to play, they sell a monthly membership for tokens used as a way to enter a sweepstakes to win the prizes. The sweepstakes however, is still the same poker tournament/ gambling game.