The missing perspective here is Overwatch isn't very effective. Blantant cheaters are easy to find but anyone with half a brain can use certain types of cheats to remain easily undetected. The only solution valve has to stop cheaters is automatic detection through software which they do with VAC. However, "private" cheats (essentially paid for and only shared with small few, likely never advertised publicly) are notorious for never being detected.
Both may reduce some cheating but overall CSGO remains one of the most cheater infested games. The latest approach by valve is something called Prime matchmaking, where you must verify your identity with a phone number in an attempt to reduce throw away cheating accounts. This further divides the user base into smaller groups.
Many of the problems with cheating stem from the data passed between client and server. Though many of these design choices have advantages for other client performance/features, they allow cheating to prosper.
As a side note, third party match making such as ESEA use a more extensive and supposedly more effective anti cheat software. These services are usually paid as well.