I think a lot of this comes from Riot's original marketing system, back when the game was still small and they needed to pad out play in order to get players to stick around for a little longer. There wasn't such a heavy mastery difference between newer and veteran players back then, so the barrier to entry wasn't quite as huge either. Now, though, the game is so chock-full of meaningful content and replay value that, even if literally every champion, rune and mastery were handed to you from day 1, you'd still have enough diversity to last for years, and that's not counting the game's regular cycle of additions and updates.
My opinion on this, though, is that it's clear that the new player experience in League is abysmal, in part because it's still made artificially more difficult than it already is through archaic legacy features. Not only do newer players start with a stark difference in mastery, in a game that punishes you harshly for making mistakes and not knowing a broad body of information, they also enter matches with significantly less raw power than the average level 30 player, meaning they're essentially set up to fail over and over again. That's not particularly encouraging, especially to someone who's on the fence about committing more time to League on their first run. Ultimately, I think part of the answer should involve getting rid of runes and masteries, which used to be standout features, but are now obsolete and far more trouble than they're worth, but even now there's a lot that could be done to lower the barrier to entry, or at least avoid punishing players for making the "wrong" decisions, such as removing Tier 1 runes, giving players a starter pack of "essential" runes, better tutorials, including specialized tutorials for jungling, etc.