Because most people in these businesses are not remotely tech savvy enough to use anything slightly different from the computers they are used to, which for most people is windows. They don’t want to tinker, they don’t want to learn something, so they’re not going to. They want to set it and forget it.
If they hired someone like you to come set it up, they need to feel comfortable editing/modifying it. They’re not going to contract you forever.
Simply put: they want to do a little as possible and they are probably being told from people several levels higher that they will be getting a TV with a computer hooked up to it to handle all their menu. They probably have software or something all set up for it. No one would ever question it, least of all a franchise owner.
On an individual basis, sure, I imagine somebody could and maybe even should do what you’re suggesting.