When small leagues get caught rigging games, they lose credibility, so do you really think it never happens in major leagues?
100% they are not doing this since they have nothing to do with bettor. Sports leagues is not part of the sportsbook especially profit because they get their profit on sponsorship from different brand.
They have nothing to gain on fixing a match while they are risking their reputation for the sake of something that is useless for them.
May few staff and players might be involved on match fixing but it’s hard to imagine a while league doing this.