Armor & Magic Pen

I've been starting to notice a trend In my recent games of assassin's stacking armor or magic penetration to the almost the max they can seem to get. For any champion that has no block or invulnerability this makes it quiet hard to play. Two examples are , granted I know its not impossible to stop them but, They can seem to burst down people surprisingly faster than you would expect, and there are few counters to them stacking magic pen with ap / armor pen. For example. I was playing as Malphite , and their diana had stacked With , And I was ale to get bursted from 3571 in 1.57 seconds even if I had defensive items like and It still wasnt enough Mr to live long enough to stun her