Answer is quite simple. This game is not all about damage but mostly about game play and champion design tho. Kha'Zix is by far more superior than Rangar since Rengar is 1 trick pony, you will get feed and dominate or get behind and suck balls. As Rengar, If you don't have your R ready to use before the team fight you are simply useless and also, Rengar doesn't fits poke team combo.
In the other hand, Kha'Zix is more versatile in terms of literally everything (except damage ATM) comparing to Rengar. You can use your Leap whenever it's off CD while Rengar can use it only if he's in the bush or out of his Ultimate. Kha'zix's Void Spikes are great tool for Poke team comp while Rengar's bola deals minor damage and is lower range and is mostly used as a single target skills hoot CC. Kha'Zix have 2 or 3 stealths to juke his enemy or proc his passive or engage or disengage, Kha'Zix can proc his passive whenever he's invisible and he can be invisible more often than Rengar can, etc, etc.
Kha'Zix was always the problem for Balance Team simply because his kit is too versatile comparing to Rengar who's nothing but a brute all in force and If Rengar doesn't have enough damage he would be useless. Kha'Zix evolution system is problematic and everyone knows that when his Q was strong people evolved Q and he was able to hit like truck, when his W was strong people evolved W for insane poke on low CD even as AOE and If I remember well Kha was able to use his W in mid leap which was devastating, his R offered him defensive stats on top of stealth, unit collision and speed buff and whenever something is strong on Kha'Zix he would be on the top of the food chain while Rengar simply won't.
Don't get me wrong, I like both of them, to be honest, I even like Kha'Zix more than Rengar since I love insects even irl but yes, Kha'Zix's kit is more superior, harder to balance and simply more superior than Rengar's and that's the main reason Riot keeps Kha'Zix under the edge right now.
Rengar is indeed more viable than Kha'Zix right now but it doesn't means that Kha'Zix wouldn't push Rengar out of the rift after a single serious buff to any part of his kit. Actually, the only thing Kha'Zix needs is his isolation range to be reverted to pre nerf state and he would be 3 times better than Rengar. :)
They are quite similar, both, Rengar's and Kha's kits require high damage to be viable but as I said, Kha's utility is superior and if he would be able to deal the same amounts of damage as Rengar he would be able to abuse the combination of insanely good utility and insanely high damage potential which ain't that good imo :D - Looking at you Lee Sin.
from running through my team (Through 
) And STILL one-shot me and he gets away. I would have won a recent game if Rengar wasn't able to one-shot me every teamfight.
In my experience, the best alternative is to just keep killing people. No reason to stop at just one.
really holds her back imo.
with thunderlords hydra and the burst is insane
