You link me a site where no one has more than 3 items tops on rengar and most have 1 or two.....dude that not how you compare builds.
That's how you compare early game.
And even the Diana players you linked never grab more armor than zhonya and ninja tabi, and that's against a 4 AD champs team in that case.
But even the Diana players there clearly prefer high AP builds if at all possible.
Check OP,GG for what the Koreans build and use on Rengar and you´ll realize that black clever is the third most bought damage item, because it applies on abilities as well.
The 300+ games rengar mains there aren't stupid by any means when it come to league, something you ought to realize if you had the presence of mind to realize all of them are sitting between diamond 2 and masters.
And you´d also realize that while they do build a ton of AD glass cannon style if snowballing hard they also frequently build stuff like amor or magic resistance like Randuin or spirit visage,because that's legit on rengar.
Something they never do on Diana.
And what the heck are you on about with "Because by raw stats and data - Diana is tankier with a shield"
Didn't i already tell you? They have basically the same stats and growth but Rengar's W is superior defensively compared to Diana's W, vastly so.
And if you compare their FULL builds rather than their first 2-3 items you´d also come to the realization that Rengar builds tankier.
Happiest Diana players, even on the site you linked, like to rush
into
. Then more full AP items like void staff, rabadon, lichbane while skipping zhonya for a banshee if they can get away with it.
Tanky build? With 300 health extra? Didn't you say Clever, which offers 100 more health, doesn't make you tanky?
Fact is, Rengar doesn't need burst as much as Diana does and he gets away with building tanky, the Koreans clearly know this and build accordingly.
And you would too if you weren't looking at a site checking the statistics of people that just got spanked over by the very Koreans.
For the last few years, that has served to prove that there hardly any point in checking any other region for builds or strategies since the Koreans clearly beat all the other regions by a longshot.
There is really no point in checking builds used in europe and NA if korean statistics are also available, you are far more likely to succeed using the Korean stuff, since thats where the western pros get their builds too.
Ever heard of the phrase "boot camp in korea"? it's mention by the analytics and commentators every time they talk about a LCS team having practiced somewhere.
And for a good reason, no other region has so many good players, that´s indisputable, thus i highly recommend you check their stuff since it's far more reliable than our own.