Why do AD assassins get access to so much, and AP so little?

Night's Edge gives them a 7s long Spell Shield, whereas Banshee's Veil you have no control over at all, which they must channel to activate, yes, but they can move while channeling this and if interrupted, is put on a 5s cooldown, and has a 40s cooldown overall, similar to Banshee's Veil.

They are able to stack so much Lethality, which continues to scale into mid and late game, while AP champions are left with one Magic Penetration item, two if they pick up Morellonomicon.

They get to purchase Guardian Angel, which gives them a good deal of damage, brings them back from death with 50% of their base health and 40% of their maximum mana and provides armor, on top of giving them a Stopwatch that they can use one time, thus giving them a miniature Zhonya's Hourglass effect.

They get a retro-styled Deathfire Grasp for AD, Duskblade, which can slow enemies for 99% (even if it's just a quarter of a second, that's insane). Couple this with Youmuu's, another Lethality item which speeds them up, they can stick to their target easily (especially considering every assassin are packed with mobility).

Meanwhile all AP champions get is Morellonomicom which gives 15 magic penetration flat and... Grievous Wounds. That's it...

Granted, mages having access to more magic penetration would make LeBlanc a lot more problematic as it is, but why is it that AD champions get so much access to Lethaility, and Mages only get one option? Shouldn't AD's access be just as limited, or AP's just as free? I'm probably just being a butthurt baby about it, but it's ridiculous seeing how much babying AD assassins get year and year and yet, when a mage is too bursty, it's a huge problem. I'm also tired of seeing all these Resolved traits getting nerfed, and yet Electrocute and so many other problematic runes that are making champions burstier than they already are, are ignored and given the blind eye.