Making a trailer door is easy. Making a high quality insulated door with integrated screen door is much more difficult. This is one area where I would rather buy than make. I don't even understand why seemingly all of the option are so small.
Making it is one thing, fitting it is another.
I looked into enlarging a window in my camper to put a window unit in it.
To do it right I would have to blow the wall apart, widen the studs and then reassemble the wall.
As far as why they don't for things that theoretically will take a beating like an expedition rig... the bigger the door the less structure is in the wall.
Lol, I have to turn sideways AND duck to get in mine...