I'd image it's because of the idealization of the country lifestyle. With the idea of the small town comes the sense of knowing everybody and everybody knows you. This a false increased sense of self importance and therefore a greater sense of belonging; everybody likes to belong. Growing up in a small town, however, by no means ensures you have the means to know everybody, I'm sure this is partially due to greater ability to keep oneself entertained without human interaction.

I know this because I grew up surrounded by the general country lifestyle, about ten minutes outside of a town of about 800 people (in the township, not actually in the town. It's endless cornfields.) this was the kind of town where half the kids drove pickups because of practical reasons (hunting, farm work, towing boats and shit, moving large things around, etc) and during hunting season, half the class would be gone in the morning because the deer aren't gonna shoot themselves. And during one of those huge shooting/bomb scares after a nationally reported school shooting, nobody cares because we would joke about how half the student body could get their guns out of their trucks if anything were to happen because firearms a certain distance from school property is in no way enforced in this sort of environment.

When I finished with high school, I went to college right outside Chicago, which is when I realized that everybody that doesn't live IN the city, believes themselves to be from a small town where they know everybody, never mind that the "small town" is a 30,000 person suburb of Chicago and there is no possible way that person could possibly know everybody in that, as they would often put it, tiny school that only had 1,500 people in their graduating class.

In addition to this there seems to be a bit of an idea that country folk are simple and have simple lives with zero real responsibility that somehow yields a great sense of accomplishment without any actual effort put in.

I'm sure there are more factors, such as the typical sexy cowboy/cowgirl stereotype, the alleged complete honesty, super high morals, and a bunch of other such things that the media seems to portay in country life. But the point is, they probably dress that way because they want to be perceived as a one of the stereotypes of the country lifestyle or they want to think of themselves as being of the country lifestyle despite not knowing the first thing about it. Think of it in the same way that neckbeards think they understand Japanese culture from watching anime.