There exists big sports betting exchanges like Betfair, where there isn't a "house" and where they don't really have an incentive to limit big winners. I have friends who make consistent profit from these sites - though it must be said they do ramp up the commission
How do you know that they make a consistent profit? You take their word for it? Gamblers are notorious liars and are also known to keep very poor record of their bets.
But that's not true. I know several guys who make a living at it. The casinos don't care because they make their vig on the action. The only losers are the folks on the other side of the bets.
It is true that the casinos will find a way to ban people who find an advantage in traditional games like blackjack (think card counting), but that's different. In sports gambling, the profit is extracted with the vig/spread.