I have been guilty of "kill stealing" as the term goes... *coughScathecough* But all competitive games which include K/D/As have this problem.

You say that if you do 99% of the damage that the kill should go to you, however, what if you spent all your resources to do that 99% and have nothing left to get the kill? Do you think you deserve the kill despite not being able to actually finish the job?
Would you rather the enemy get away with a sliver of health or would you like the enemy to die and the kill go over to anyone on your team?

There are some instances where you do most of the damage to someone but you actually have no way of getting the kill either. It's better if one of your team members gets a kill than none of you get one at all. Also there've been times where I've let someone take a solo kill but the enemy actually gets away.

Not to mention that what if you did 51% of the damage to someone? I wouldn't think that you deserve the kill since someone else might have done 49% of the damage. You both didn't deserve to get a kill by only half killing someone do you think?