Why don't more champions scale with stats other than AD and AP.

So, tanks commonly have abilities that scale off of MR, Armor and Health. Pyke's abilities scale off of both AD and Penetration.

What's stopping anyone from making Crit Marksmen stronger by giving their abilities more damage or better effects when they buy items that give Attack Speed or Critical Strike chance. Yasuo's Q scales off of Crit Chance, it's cooldown get's shorter.

Why not make all assassins healthier by making their abilities scale off of Penetration instead of just flat AP, that way if your being one shot by a Talon it's not just because he's building a certain item that happens to be extremely over tuned.

I just don't get why more champs don't scale with the stats they need to get, i understand wanting to keep a champions options open but at the same time if a champion has too many options or if a class does those champions make it worse on everyone else, and considering how often Crit ADC's have built Bruiser and Assassin items because their just flat better for them while also taking into consideration how many Assassins have become tanks i just don't understand what's stopping riot from doing it this way instead of the current way.