Do you realize how long that would take?
How is that any kind of fair at all that you go into champ select and you might be like "Hey I feel like playing hecarim today" or "Hey, hecarim would fit in this team comp really well" but oh wait, you don't have level 4 so you can't play him.
Also having a certain mastery level with a champion DOESN'T necessarily mean that you're suddenly a god with them. All it shows that you have played and are familiar with the champ.
How would you feel if you were mastery 7 with a champ and you went into a ranked game and got dumpstered and fed to someone that was mastery 4? What sense does placing mastery restrictions have when everyone plays the game and champs differently and the mastery level is not exactly an indication of player skill level with said champ.