Why not pressure treated for floor subframe?

I've yet to see a build thread with people using pressure treated lumber for the subframe of their floors on their teardrops. Why is that?
Is it a cost thing? Is it a chemical thing? Or do people just assume that because they are covering it in CPES or undercoating that it'll be fine?
Im just curious. Is there some other reason? The subframe wouldnt be inside the sleeping quarters so I cant see fumes being an issue. Am I wrong in thinking that any extra protection from water on the bottom of the trailer is a good thing? Can pressure treated lumber be treated with CPES after for even more protection?
Is it a cost thing? Is it a chemical thing? Or do people just assume that because they are covering it in CPES or undercoating that it'll be fine?
Im just curious. Is there some other reason? The subframe wouldnt be inside the sleeping quarters so I cant see fumes being an issue. Am I wrong in thinking that any extra protection from water on the bottom of the trailer is a good thing? Can pressure treated lumber be treated with CPES after for even more protection?