> Why should something that goes through a town be a highway?
Because the town wants to be as close to the highway as possible, which in practice means the town gets built on either side of it and the highway ends up going straight through the town.
If you moved the highway outside of the town then the town would just move over there because the businesses next to the highway get more business.
I'm not sure if that's true. In my country, most towns don't have a highway that run through them, and there are no calls to change that. People like easy access to the highways, but that means easily getting out of town to get to the highway.
Nobody actually moves next to the highway - it's mostly workplaces that are next to the highway so that employees can easily get to it. Businesses that have regular people visit as customers, however, are where those customers are, which is where they live and walk around.
You've getting to the root of the problem in the US: The zoning doesn't allow businesses where a lot of people live because it's zoned exclusively for single-family homes. The "town" is mainly businesses, and they want to be near the highway.
Because the town wants to be as close to the highway as possible, which in practice means the town gets built on either side of it and the highway ends up going straight through the town.
If you moved the highway outside of the town then the town would just move over there because the businesses next to the highway get more business.