Can any Rioter explain why Mastery 6&7 are hardly related to actual champion mastery?

The champion mastery system should be about skill and performance, not about luck and purchasing chests/keys/etc.

Mastery levels 6 & 7 seem to be just another incentive to spend money through the Hextech Crafting system in order to get your champion shard. The chances of someone getting the exact thing they want WITHOUT purchasing things is extremely low due to the absurdly low drop rate of key fragments.

Riot, I'm not upset that you're trying to make more money from your game. But don't tie it into champion mastery. Please.