I have an office job that entails a lot of interaction with people who in the construction industry, small business owners, developers, bankers, etc. I'm too androgynous for my employers to feel comfortable with letting me meet people in person (thank god) so most of it is through email or phone calls. They all vary in education and income; some have PhDs, some never finished high school, some are rich as fuck, some are struggling to get by. Most of them are local but I work with quite a few people from different parts of the country.
There's something that is common between a lot of these people, maybe even the vast majority, is that they cannot do extremely simple tasks or understand simple concepts, even when I try to explain them visually (like I'll share a spreadsheet with them and go through each individual thing I'm doing to show them what I mean). Very few of them get it. I'm not particularly smart or amazing at math or anything, but I'd like to think I can understand simple instructions. Sign this, add these numbers, make this match this. I can't imagine what it's like working in retail if the average person is this dumb.
I'd say it's a bias of your job. To you doing the spreadsheets is dead simple because you do it all day. There are things that are second nature to you that other people struggle with. You're judging them by their ability to understand your work. There are things they do that you would also struggle with but you are never in that position so you don't realize it.
Everyone is stupid at most things.
Construction is a very friendly field to people who struggled at school because you don't have to be book-smart to do well. It's also full of men, which means a lot of arrogant assholes. A significant amount of the people who don't understand spreadsheets are just prideful middle aged men who think "book learning" is beneath them and get all pissy because they refuse to learn from someone they see as beneath them (some of them literally cannot read and are too embarrassed to admit it as well).