Because that is terribly boring.
Even 1 encounter per short rest as it was in 4e was a boring assumption.
Combat felt as if you had to always spend everything in the right order or you did something wrong.
Felt kinda like CRPGs where often you had a standard routine of oressing buttons.
I know I will get pushback for this comment. But that feeling grew by playing 4e for several years, not by just reading the rules.
4e wasn't even balanced around encounters per day. 4e was balanced around healing surges.
The point was that you need some sort of resource change or attrition or every combat would become repetitive as you said an action CRPG.
----
Here's the thing I do get?
The median on the poll is 2-4.
5e was not designed to only have 3 combats regularly. Sometimes. But not common.