You already figure it out from the pro side, but it sadly is not enough on the casual side as well. The issue is streamers would show their audience how easy it is to cheat and this would lead to a perception by more casual gamers that cheating is rampant and so they would be less inclined to join. This reduces the audience, hitting worse match making (as you suggest), lower stream viewership, less game success for the studio, etc.
As a pretty bad FPS player (was ok on consoles but never got the knack for aiming with a mouse in the same way), matchmaking works well enough for me. My better friends are always complaining about hackers, but I almost never encounter them.
Downside to all of this is it would have to take longer to get into matches to keep players in their pools. I know games already do this, but they seem to match players up or down too quickly to keep the velocity of matches going.
As I type this I realize this system wouldn't work. Pro players would complain about long waits whiles average players would play the most frequently. Lose pros and you lose average players.