Among all the other reasons that people have explained that this isn’t exactly correct a probably isn’t entirely attributed to just weight… I’ll make an opposite proposition.

Perhaps we are evolutionarily programmed to avoid people with impulse control issues?

I think there might be something here. I believe people flock to bigger, stronger and healthier people as some sort of evolutionary strategy for survival. If we were in a crisis and had to revert to tribes requiring everyone to pitch in to save humanity would you rather have the person who can resist impulses and keep a strict schedule (fit/healthy) or the person who looks like they lay around eating everything in sight. Using looks alone someone may assume the more healthy and fit person would be better to keep in their circle and thus prioritize being in their good graces.

I understand that it's not that simple, and someone's physical ability has nothing to do with intelligence in the real world. Unfortunately we're all subject to making split second judgements when interacting with strangers and as a result people don't think deeply about how that impacts those they deal with (or don't care).

We are evolutionarily programmed to like attractive people, which excludes most visibly overweight people.

Nope, it's cultural. If you look at cultures outside of your narrow window (in both time and space), you'll find that the perception of fatness has varied wildly over the millenia.