J.Quondam
CR 1/8
Because that's the easiest aspect of the game to quantify using a set of rules derived from wargaming roots?Then why is so much design space given over to combat and combat abilities?
I sincerely doubt that most people play the game -- or have ever played the game-- for the specific purpose of "killing others." If that were the case, there'd be no interest in, for example, mechanics to flesh out "Exploration Pillar," or questions about handling romance in the game, and so forth.
People play D&D for the same reason they play other RPGs: to engage in some higher-order story involving characters in an imaginary world, of which "killing others" could be major component, but probably is not "the central conceit."