I doubt that the design of molded fiberglass trailers meets the fast production-line manufacturing criteria of the big companies.
Although molded FG units can still leak at window seams or door edges or plumbing like any unit, eliminating the roof and sidewall joint seams helps greatly. When water gets into a stick built trailer, one can quickly have hidden mold in walls, delamination, and rot. When water leaks around the window of a molded FG trailer, one mainly gets wet cushions. (But if the leak is neglected for years, floor rot still can result.)
Come to think of it, maybe the big companies see the potential longevity of molded units as a liability to future sales. Planned obsolescence boosts sales.
