Why does it still cost BE for me to unlock masteries on my champs?

Look this may be a stupid forum to post but I'm curious on to why this is still a thing. since they took away lp and turned it into BE, took away buying random champ shards and being able to get BE after games. The only way for me to get more BE is either to grind out the game and level which causes me to tilt and not have fun ( I like taking breaks in between games if I tilt), Or Buy cases which I'm too broke to do. I mean I already spend 20$ on the new vayne skin and I cant spend anymore.

- Dr Shauna Vayne