Damage is WAY too high right now.
The reason everybody is building tank is because if you don't, you will die from one combo and obviously you won't get your damage off then. But you can build full tank and still do the same damage as you used to be able to in seasons past and not blow up before you get a chance to use it. You either have enough range to outrange the other team, or you build really tanky so you don't die immediately.
Let me explain, I play Renekton, I've mained him for years, ravenous hydra has always been CORE on him. It synergizes better on him than 95%+ of the champion pool because he can use it to animation cancel his stun and he has always been a BRUISER who benefits greatly from the AD and lifesteal. But guess what? I find myself almost never even building the damn thing because I have to get more defense so I don't blow up. And I still do as much damage as I would have in the past with the hydra.
And of course this unbalances the hell out of any champion that loves building the meta tanky items right now. Oh, and it's also not bad for those certain champions that are impossible to trade with because they have zero interaction laning because they poke you out of lane even faster.
Why?
Honestly I'm not sure. If I had to guess, it's probably keystone masteries. I don't think they should have ever been added to the game.