Because making money doing fraud when running an exchange is easy, and making money without doing fraud is hard.

You accept cash from credit cards with risk of clawback etc., transaction costs for money major cryptos (bitcoin, ethereum) are through the roof, you need lots of users to make a liquid market plus cost of infrastructure and development.

I don't see how an exchange is making money unless it charges ++% pr. transaction or does something that invites fraud and sudden collapse like keeping all crypto in a single wallet.