I want more penetration

Magic penetration seems like such a lackluster stat to build when compared to lethality. With magic penetration your options are very limited and you can't really stack it like lethality. To me it feels really lackluster and the extra penetration from mask is hardly noticeable. Is there a reason that magic penetration isn't a large part of the game?

I wanna feel like zed or talon does when they do true damage to a squishy because they stacked lethality. Even just one more flat magic penetration item that is offensive would be cool.

I JUST WANNA PENETRATE DAMN IT