>>fallin+(OP)
Much, much fewer people would gamble if you had to do it by finding some weird person and handing them cash and trusting them to run a fair book, than just clicking some buttons on an app. After all, that’s why they’re apps that are constantly advertised; gambling services don’t have customers they attract with offers on the free market, they have victims who’s better sense they overcome through convenience and manipulation.
Black market bookies also would see consequences from getting caught rigging a sports match, anyway. For one, they would be punished by the law for being black market bookies.