If the reputable gambling website cheating the members, and the members know, they will get into the problem, and the member can chase the site with help from other people who know about the law. Yes, we don't know if the system cheats or not, but we can anticipate from the cheat by not using too big money for playing gambling.
But isn't the point of the Provably Fair system just to know for sure that everything is fair? As I understand it, any player with average technical knowledge/ability can check the fairness of the game on the history of their bets. I read that there are certain nuances and "loopholes" but in general the system works like this.
Yes, they can check the fairness, but not all gamblers can do that, and some of them are only want to play gambling without thinking about fairness or not. But they will search for the fairness of the game on that site by reading many sources, so they don't have to worry about fairness. It is good for people who can check the fairness of the game, so if they find something strange, they can show to people, and maybe they can get help from other people who can check the fairness.