I don't wear my pjs on an airplane, and always try to look nice/presentable. But in defense of those who go super casual, and to answer the oft-heard lament of why people no longer wear their nicest clothes while traveling (as they used to, apparently, in the 1960s): most people probably don't think anything of wearing wrinkled clothing on an airplane when they know they are going to get on the plane, be stuffed into a seat that is tiny, end up sitting between two people who encroach on their space, and have to take off any jacket and wad it up under the seat in front of them or shove it into an overhead bin. I'm not sure if the size of airplane seats have gotten smaller since the days of high fashion travel, but they haven't gotten any bigger, while the average size of Americans has. So that means there is more size fitting into (at best) the same sized space.
The perks that used to come along with air travel, like meals (sometimes with actual silverware!), are gone. Airlines are constantly overbooking their planes, so there is a good chance any given passenger can be bumped to another flight. Airports are bigger, so getting from gate to gate if there is a connecting flight becomes more difficult. Security is stricter, so wearing items that set off a metal detector--often anything with any kind of metal at all (including, as we have seen from previous posts, underwire bras)--can result in a delay at the gate.
In light of all that, it sometimes doesn't surprise me to see people dressed sloppily. They probably figure they are just going to be standing in line, moved along like cattle, stuffed into a plane like sardines, sure to get wrinkled regardless, and they can just deal with looking nice when they get where they are going. I also think people used to dress nicely to fly because it was a big event. It just isn't as rare in most people's lives as it once was. Additionally, I think that the airlines/stewards/stewardesses treated their patrons as if the act of traveling was a big occasion, and that isn't the case anymore either. In a lot of ways, I think it is probably partly that people don't dress respectfully because they don't think they are treated respectfully. Again, dressing like I don't care while I'm on a plane isn't something I do, but I do understand the reasons others would feel that way.
