>every time I go visit the zoo with my sister's kids I see the most absurdly fat people on the planet
>I'm talking like 600+ lbs easily for the men and 500+ lbs for the women
how do these corpulent masses continue to trudge on? their existences sicken me, and they are almost certainly subsisting on my tax dollars

I can't speak for all of the fatasses, but I do know it's a slippery slope that's really easy to fall down on. It's especially so when you're depressed and alone all day, only going to work and coming home. I just kind of woke up a decade after failing out of college and was that with no skills to show for it. I'm making a large effort to undo the damage I did to myself but it takes time, a lot of people give up because the effort to fix it is actually a lot.