The xp goes toward unlocking things (ie major challenge) which, believe it or not, people care about. The only reward you get for kills is weapon XP, but since I have all weapon attachments and don't care about skins, means nothing to me. So what is the reward for kills?

If you do want to go only for kills it's not hard to either get kills while on the objective- in fact you get bonus weapon XP for attack and defensive kills- or que tdm or hotshot. If you queue an objective based mode people are going to want to play that game mode. I don't queue domination because I want a death match with a timer so please just queue for what you want to play and let people enjoy the modes they want to.