The shower stalls, like most of what goes into the construction of an RV are "insert part A into part B" simplicity...designed to be installed quickly and by minimally trained people..as fast as possible. The shower stalls, and the PEZ fresh water piping systems and pre-made walls for example.
Most RV manufacturers have it down to a science. Many Factory built RV's are made by folks who don't get paid by the hour, but rather by their output. When they reach their daily goal of (for example) 8 RV's...they go home to work the farm while the sun is still up! If they reach their target goals for building their "quota" and with out failing quality control checks..they get their bonuses too.