Why is there no scaling total or bonus AP ability?

We've seen total AD ratios scaling from 1 to 5 (like on ) and bonus AD ratios scaling from 1 to 5 (like on ) but there is no scaling total AP ratio ability or just a simple bonus AP ratio ability. Why is that? On a champion (like an AP assassin), a bonus AP ratio ability could be really beast late game. A total scaling AP Ratio ability (like a 30%, scaling to 50% at level 5) could be a great idea for a new champion.