Why are penetration items allowed to stack so efficiently

Why can't flat penetration apply before % penetration

And why does it have to be so effective vs towers and epic monsters?

There was a time when getting 40 flat pen was pretty much the most you could get, back then it didnt work vs towers and neither did it apply after % penetration but rather before it.

People where still killing each other back then, so there isn't a need for all that. but its here and that is all fine.

But you can get 87 penetration now which is more than double of what it once used to be, and it is now capable of completely removing any armor from squishy targets, as well as being very effective vs tanks because it stacks so well with other forms of penetration

isn't the fact that you can get so much pen enough? should it also stack insanely well with % pen? And should it allow you to true damage towers and other neutral objectives?