One of the big reasons why Lethality is somewhat underperforming atm.

Towers, minions and a majority of all monsters have zero armor and MR, sometimes even negative MR.

This means that picking up Lethality or flat Magic Pen is completely wasted on these targets, while stats like AP, AD, attack speed etc. are not.

Just go into a custom game and get a rune page with lethality marks and quints and try to farm minions or monsters for a while. Now do the same again but instead get yourself AD marks and quints or attack speed quints instead and you have a very noticable difference.

It kind of sucks that champions that want to buy either lethality or magic pen can't really even take it in their rune pages because they would have to give up a lot of early game power because of this. The only real exception to this are flat magic pen marks because they are the only real marks AP champions can take.

What if minions, monsters and towers get more resistances but less health? This way, lethality and magic pen wouldn't be useless against them while everyone who doesn't build lethality or magic pen would deal about the same damage to them.