The problem is that most ordinary gamblers like us have no information about rigged matches, or even if we know, we do not know who is going to win. Some bookies who are operating on a larger scale, they may have the knowledge of who is going to win and this can lead them making a lot of money in those matches.
The bet industry isn't fair, there are match fixing, rigged matches and all such stuff going on and big players are utilizing this to make more and more money.
No one would argue that match-fixing occurs; there have even been official cases where the parties admitted that there was an agreement on a specific outcome. However, those who fix matches usually do so to make money for themselves, not to enrich bookmakers, so such information usually does not go anywhere, otherwise there is a risk that the match will simply not happen, and those who organized it will face some kind of sanctions. Therefore, it is usually not profitable for the initiators to spread information about a match-fixing scheme, and if the initiators are sensible people, the information will simply not reach the bookmakers.