Because, it's partially the reason the game is free to play, and the other part is because they're constantly making more champions. Just giving people them is a retarded idea. It's annoyingly overwhelming to go into Dota 2 with their like 20 champions, I cannot imagine how overwhelming it would be to have all however the fuck many champions be free.
Go ahead and laugh all you want, but they are often balanced or even underpowered upon release.
Sig: Elyssia | DJoron is the Best
Dota2 manages f2p because VALVE ARE THE ONES BEHIND THE GAME. They have infinite money to pool from. And even then, if valve didn't give everyone a million dota 2 invites, you'd still have to buy the game itself for $30.
There is one thing that LoL could improve, and that would be giving new players a balanced pool of champions for each role to pull from, that much I know, the free week stuff is kind of stupid and it doesn't foster improving in a certain role.
Sig: Elyssia | DJoron is the Best
Sig: Elyssia | DJoron is the Best
Everyone who's played the game for any length of time already owns all the champions they're going to buy, most of the money riot makes is off skin sales. No one even buys champions with RP because the IP for them is so easy to get. It would actually be an incredibly stupid thing to buy champions with RP.