Why are runes so heavily gated?

This has bothered me for a long while, but bits and pieces of the eCommerce Q&A reminded me of the biggest thing that I dislike about LoL's rune system.

Why are there so many gates when it comes to runes?

Consider other Summoner aspects of the game. Spells are the simplest gate. When you reach level X, you have access to these summoner spells. At level 12, you have uninhibited access to every summoner spell. Masteries have two layers of level gates. You get one mastery point per level, and you gain access to more masteries as you reach higher levels. At level 21, you have uninhibited access to every mastery, and you can fully utilize masteries at level 30. Great!

And then there's runes. Like other summoner aspects, runes are level gated. You get one rune slot per level. They are also level gated in terms of tiers. You gain full access to runes at level 20, but unlike spells and masteries, runes have inhibited access on account of the final gate. The final gate is an IP gate. In addition to being limited to slots per level and tiers per level range, you also cannot use runes until you purchase them with IP. And this is while you're STILL gated to one rune slot per level.

So now that I've laid out the obvious, I come back to my question. Why, when compared with spells and masteries, are runes so much more drastically difficult to utilize? I don't really want to argue that any of these gates are particularly good or bad, but the fact that there are so many of them compared to spells and masteries, and they all together make for a system that is exponentially more complex. And it's been like this for as long as I can remember. Surely, there's must be a better way.