In 13th Age, pretty much all spells scale with level though. I don't think it actually has mage armor, but if you look at magic missile it goes from 2d4 damage as a 1st level spell to 6d6 as a 7th level spell.Or like 13th Age where high level casters lose their low level slots. So a 15th level wizard would only have 9th, 8th, and 7th level slots. So if you really wanted to have mage armor at the very high levels you would be casting it as a seventh level spell and using up one of your spells known on a first level spell.
I think I've seen one of the designers mention that the early 5e adventures (don't recall if it was Princes of the Apocalypse specifically, or the early ones in general) deliberately didn't give out many magic items because they thought DMs would add in whatever items seemed appropriate to their party. I'm pretty sure the actual books don't say anything like that, though. And I'm not going to try to find an off-hand comment some designer made ten years ago, so the source is going to have to remain as "Trust me, bro".This table doesn't even align with WotC published adventures. I have Princes of the Apocalypse (published 2015).