Why so little reaper cheese in top-tier SC2 games?

Solution 1:

Heavy reapers opening have been tried especially in the past, but also recently. Check for example Reynor vs Clem

As for other kind of cheeses, they don't work very well at the highest level because players know how to scout, how to read what they find and how to react accordingly.

If a player scout no 2nd CC and 3 rax, the player knows for sure some form of aggression is coming and from rax without a tech lab attached, it can only be either marines or reapers which are easily dealt with in the same way. The player would then change whatever strategy he/she intended to go for to counter this early aggression, possibly dropping bunkers/spine crawlers/cannons/shield batteries, going heavy on marines or stalkers or queens and lings. This rush is pretty easy to defend, no matter the race. I would be surprise this would work from diamond up.

Your fallacy is to take as an example of your thesis random TvX games and comparing their army at the 3 minutes mark with yours, in a totally unrelated game. If a player, after scouting, confirms that the opponent is not going for any kind of aggression and doesn't want to be aggressive from the start himself/herself, the player will invest in the economy and that's why you see few units in the player base.