I find the more I eat of sugary or fatty food the more I can handle. My body seems to get used to it. So I'm guessing how people do it is they slowly increase over time without noticing.

Hell, maybe they do feel like shit all the time but don't associate it with their diet.

Sugar was popular way before industrial food processing was a thing. If you look at old paintings of monarchs and other elite people, even pre-industrial food very frequently led to obesity if you were rich enough.

And you can easily get fat without any sugar at all. Starch, meat and cheese done right is very tasty.

They put less than a gram a serving in some brands because it helps with the maillard reaction. You aren't tasting that, you're looking for something to screech at.

You're also just wrong and probably not actually looking for what you claim.

It took me all of literally 10 seconds to find this and other popular frozen potato brands. The only one that had added sugar was oreida and it was under a gram.