Why do I have to own a champion to get key fragments?

Chests I can understand. But keys? Why? Like I see no reason why I should have to own a champion to be able to earn something to get something that I need 3 of to open chests. It's not like a chest where you or a teammate has to get an S- or higher to get a chest, the chance is always the same and goes down with every fragment you earn and resets every 30 days since the first fragment you got. Chests are directly linked to mastery, while keys aren't. So why should I have to own a champion to get a fragment? Maybe I'm missing something here and that's why I think this way, but if anyone can give me a good reason as to why I need to own a champion to get a key fragment I would be glad to hear it.