Why are we limited if our account is under level 30? I have a friend who is learning how to play and he is struggling to grasp the importance of runes and masteries simply because he has none of them unlock yet. How are we expected to learn the game properly if we aren't given the tools to learn it properly by being limited by level for mastery points and low tier runes that are virtually worthless?
The obvious answer for Masteries is the same reason you don't teach Kindergartner's Calculus.
Also why games with 1 obvious tutorial phase in the beginning tend to have a painful tutorial experience.
People don't learn well by having everything dumped on them at once.
People learn things much better by having 1 problem to solve at a time rather than 99.
Which easily explains why Riot would want to level-gate Rune slots, Masteries, Summoner spells, and certain game modes.
It gives new players a narrower focus of issues they need to deal with without getting overwhelmed.
That isn't to say Runes having 3 tiers and 30 level-gated slots is good.
Both together is pretty redundant, and the way runes are implemented leaves no reason for T1 or T2 runes to exist, both largely being IP traps.
Rune tiers have been implemented very poorly and don't have any real justifications in existing.
But as far as you're learning friend struggling with certain aspects of the game?
Even pros struggle with certain aspects of the game. There's so much in LoL to learn the learning never stops, which is one of the big interest points of the game.
And really - runes and masteries just aren't an important thing for a new player to grasp. At low levels they just don't make a difference, and chances are your friend will bother looking into both before they actually hit summoner level 30. There are plenty of other, more important, things to be coaching your friends on than those.