Why is MR so powerful when compared to worthless armor?

Why does so much MR exist for AD champs, when there is one single item for armor? ( Speaking Mid-laners) We have / , , all providing significant MR, yet only / for armor.

Then we look at penetrating the other one's defense. , , /, , for a total 45Lethal + 30% shred + 45% bonus pen.

And for mages we have a whopping . 30 + 35% pen.

This isn't even including the vast array of champions who can debuff armor vs the 5 champs who can debuff armor and only one of these five being a mage ()

The other main issue is DPS v burst. Mages generally have high burst but then drop down to a very low damage output. This means if they can kill their target in a rotation, they win. If their target survives, they have to retreat until cooldowns are up. AD however, usually have burst, but also have semi- strong auto attacks. If their target doesn't die in a rotation, they just beat their face into a pulp with some AAs. Having such easy access to magic resist, all an AD needs to do is buy a little bit of MR to survive the initial burst and then retaliate hard while the mage is on CD. They can also benefit from lifesteal during this time, so even if they cannot kill the mage before cooldowns are back up, they will be on a higher level of health than they were before. Mages don't get this luxury.

**Since mages are ENTIRELY reliant upon their skills, (AD have high damage skills and high damage attacks) is there any wonder why we constantly have a full AD meta when the items are so tilted?

Can you even remember the last time all AP was meta?**

P.S: Late game it isn't as big of an issue, since Void Staff is pretty strong (scaling from total MR rather than just bonus but that also means it isn't a great early item), but all through early/midgame the difference ridiculous. Sadly most games are decided by then.

EDIT1: Read the body text, not the title.