why do tanky junglers/top tanks in general do as much damage as they do?

i cant get this over my head, every time i play it's the same thing over and over again. i get it im new, because i wont lie, sit here and say that i am a diamond player, i am a silver 4 player that basically just strted playing ranked. but the thing that i always see is these tanky, and i mean TANKY junglers/top laners that build full armor/hp and still manage to get a ton of damage across, like for example rengar, im not calling him op or anything but jesus, every time i go against him it's the same thing, full tank build with little to no damage at all and jumps, slows and bursts down a ton of hp. or renekton even, full fkin tank and still does a load of damage, i know everything has its counters but this looks ridiculous to me, they manage to get so much damage across even with their full tank build.