Try to build an 8 foot by 25 foot cabin with bed, bath, kitchen and the associated plumbing, electrical, lighting and appliances, for $25K to $28K.
Now put wheels under it and drive it down the road at 60 mph. then off road for a few more mile.
Oh, also,it has to weigh less than 6000# and you have to charge for your labor.
Lots of folks have tried and most of them are now owned by Thor or Forest River.
It is tough to do and even tougher when folks do not want to pay extra for a quality product.
The higher priced unit may use plywood over pressed board or better construction techniques and appliances but the manufacturing process is pretty much the same.
The Casita/Scamp egg types or Airstream are a few of the non-standard construction types.