Because that is the standard "Template" of any shooter, KD/A is the most prominant stat because it's lowkey what every game does, only a few games have a damage counter tbh.
KD/A is like.. "Raw Infomation", i mean why do you think everyone loves to show the Damage badge, because the badge gives your teammates an rough idea of your contribution to gain kills, the kill it self is the teams goal to prevail, but said target cannot be archived without..well dealing damage to the respective target.
The kill is the result, the damage the way to the finish line, take damage more as a "Indepth" factor, than just mediocre numbers.
A case:
You play Duos with a random (A) and you are (B ) - The end of the game, whether lose or win, you have 2.8k damage, among that 2 Kills/8 Assist/2 Knocks
Your mate (A) has - 8 kills/1 Assist/8 Knocks with an overall damage of..meeh let's say 900 to 1.1k.
Do you understand what im saying? The damage is ultimately the very fact of how much of a contributing variable you are in your team.
We could of course make (A) an saboteur, he waits willingly till you crack the shield and afterwards starts blasting to get the final blow, does that last hit in your mind really make a *click* and be like: "Yea that was all you! Good job!"?
You tell me now who contributed better to archive the target, does any of that make sense?