is ''damage creep'' to blame or defensive itemization for so much damage?

just think about it, before maw of malmortius and mercurial scimitar mages didn't need as much damage because adc's weren't walking around with 100+ MR and a 400 health shield, then you add in black cleaver for another 300 health and it seems clear to me why we need so much damage.

then there are mages walking around with 3k+ health while not sacrificing any damage for it.

it seems to me damage is so high because of all these amazing offensive items which grant superb defensive stats.

of course I could be wrong but this is simply how it looks like to me.