why does miss fortune have a crap ton of health?

I'm not sure I'd call it a crap-ton. It's ~15% more than some of the squishier ranged champions. Enough to take note of, but not enough to make a huge difference.

As for that particular design choice?

I can see why Riot would do that. Most marksman seem more adept at playing the "don't die" game than MF.

578 – 2023

Seems to me she's slightly above average, but that's probably due to her nature as a lane bully. She's not a hypercarry, she's intended to have a strong early game.

It won't matter when she's caught, because she has very little chance to live compared to other safer adcs like Vayne, Caitlyn and Tristana which all scale better.

She has a lot more "cushioining" in the front.