The only way to be really fair is for everybody to Stream the game at the same res, frame rate and latency.
If you are asking why games like counterstrike don't have limits on online play, that's mostly a commercial question. Would those games be as popular if they limited performance to what was achievable for minimum specs? I certainly wouldn't want to play at 1920x1080 on my nice widescreen monitor, but setting the minimum to a $1500 monitor and the hardware to drive it would guarantee very few players.
[0] https://www.speedrun.com/fallout_4?h=Any-Full-game&rules=gam...
Edit:typo
Some games do impose limits though, for example Overwatch doesn't allow you to use an aspect ratio larger than 16:9 and selecting a wider aspect ratio actually cuts down on your vertical field-of-view rather than granting you more horizontal field-of-view. This lessens the potential advantage of ultra-wide monitors.