Why would they tell you this? It does not benefit them and most people know. When they designed the game they probally sat around a table and listed things they had got addicted to in games before, and put as many in as made sense. Its a smart idea.
Naw, people play Dota 2 all day and that has no ranked and all unlocked.. The reason league has a grind is so you'll buy RP out of impatience. Just look at IP/XP boosts. Runes only cost IP so a resource that could be used to buy champions is put somewhere else, again solved with Riot Points.