Because this is no military simulation. It is part of the fun to know when you made a kill or not, also it is helpful to know if you just wounded your enemy or killed him.
It is midway between fun and realism. This way you can optimize the game to your liking and others who want hud elements for example or want kill messages keep them.
But then you're at a disadvantage. After all it's a competitive game and not a cooperative one. Forcing it server side would mean everyone plays under the same circumstances.
That I agree. I see the CMP server removed the hit marker, maybe one more step?