Why is there no attack speed per level runes?

Because a champion's attack speed naturally advances per level at a consistent rate and by the time atkspeed per level runes would be worth anything, it would be way too late into the game. Runes are intended and balanced more for the early game where the micro stats are more valid. When you get into mid and late game where stat differences vary more wildly depending on how everyone is playing, then they stop mattering as much.

There are plenty of "late-game" scaling runes like HP per level, and AP/AD per level, but cases being made for those being any better than flat runes are kind of hard to make. Min-maxing isn't really much of a concern when you get late into a game.