Because that's the easiest aspect of the game to quantify using a set of rules derived from wargaming roots?
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."
Combat is, by far, the "central conceit". The hint is the page count devoted to combat.
Had the game been chiefly about exploration, that would have taken up the big part of the book. Same with handling romance.
Just because you can handle romance with D&D does not mean the game is geared toward it. (All I can say is that D&D is not
actively suppressing romance, but that's hardly great rules support, is it?)
D&D is about combat. If your games don't revolve to a significant degree around combat, cool. You're just not using the full potential of the product you just purchased, and you might want to look into other games if that's a concern. (For many of you, it is not. Which, cool. Just don't say the game isn't about combat just because you chose to skip maybe 2/3rds of the page count...)
I sometimes hear this sentiment: "D&D is not just about combat". And for the person saying it, it might well be true. But just because that is true in your game does not mean it is objectively true for the published product. I would say few games are MORE intensely focused on using combat to resolve challenges, so this sentiment has never made much sense to me.
(Of course it is true in a literal sense. I mean, even a game 99% about something, is technically not just about that something, since it is also 1% about something else)
Maybe the person speaking is genuinely unaware of other games, but there are many ttrpgs that are indifferent to, or actively uninterested in, combat (the physical/magical kind), or perhaps offers a general conflict resolution mechanism where combat is only given equal treatment to, I dunno, intrigue, subterfuge, seduction or something. However, they are nowhere near as popular as D&D, and many people use D&D for those purposes.
Even though D&D is about combat.
I remember when 5th Edition came out and WotC tried to sell the spiel that there were three equal pillars. My comment:
just because you say it does not make it true The game still devoted the main chunk towards combat, and the three-equal-pillars talk? I consider it just marketing.