current state of damage in lol

there is something i don't understand. why there are champions, that are extremely mobile, hard to catch (hard to kill, aka zhonyas) and still dealing tremendous amounts of damage, and why there are champions (all immobile tanks, garen, nasus and so on), who dont do damage (becouse it's impossible to reach backline of enemys with current amount of hard cc and slows), but can't even tank, as they die in less 1.5 second while having 6 pure tank items if could anyone explain me, why lol is becoming more and more offence-only oriented with almost every patch (since season 3 when runic bulwark was removed and tank items nerfed), while 6 tank items champions can't even stand still for 2 seconds, not even telling about doing any damage or disrupting enemies and this is how it looks at diamond/master/challenger tier, no idea how it is in lower elos

thanks for answer, becouse i can't find an answer why would you make immobile tanks so useless as are they now. mobile versions aka sejuani or gragas have cc at least, but immobile tanks have literally nothing

regards

edit: of course i'm talking about late game, early and mid game it's possible to survive for at least few seconds, but there is a massacre when damage dealers reach their items. i believe the problem is defensive items, it's quite easy to reach them as they are cheap, but they fall off really quickly as they aren't even close to be as powerful as late game offensive items, so they mean nothing with amounts of ap/ad/penetration that ap/ad champs get