Right, they have items that counter them, something squishy targets can build to help them out against assassins.
So for Rengar just buy a fucking pink ward and upgraded sweeper and pay the fuck attention! Most of the times I do end up getting an assassination on a target without being in a teamfight already I literally walk around their teammates who all get the exclamation point and most of them do not even react! I can bet 99.99% that the people complaining about Rengar are in this group of people, they simply are not paying attention.
As for in teamfights, 99% of the time the Rengar won't be fed enough to instantly 100-0 without a warning, those are outlier situations that Rengar haters like to focus on, so when he jumps in he can be cc'd to oblivion, just a fact, no matter how many videos of a fed Rengar several levels ahead of a feeding adc who isn't paying attention instantly killing said adc won't change the fact that those are not even 5% of most games with a Rengar.
I can easily raise you a Riven or Katarina 1v5'ing for a penta, a Talon instantly deleting 2-3 targets as fast as those Rengar vids, a Lee Sin hitting one Q then just R for an instant kill, or a Diana flying in and 100-0 killing multiple targets who can't escape her multiple dash resets, I even saw a vid recently of a full AD Darius literally kill squishies with one auto. They're OUTLIERS, so they don't get nerfed because that would be utterly ridiculous!
And I see you're trying to make an argument that all the other assassins have item counters or they don't do enough damage to be considered "100-0 possible." You forget that I'm speaking in the same terms you have been for Rengar this entire time! When every single one of those champions BUILDS FULL DAMAGE and then GETS FED, they can all 100-0 a target, but the majority of them all have ways of escaping afterwards, something Rengar does not have.