Because people will pay for apparent convenience. The motorhome market is largely retired folks with lots of cash who want simplicity and have been made (wrongly) to think that you need to pay lots for it. Plus there is fashion cachet to be had from having an Oyster or Kathrein on the roof.
The advice given in satelliteforcaravans is excellent. I use a s/h Sky box, minidish and tripod in France, Spain and Portugal with my motorhome and it works great. Total cost about £70. Plus there is the added advantage of being able to locate the dish in the open even when you are parked in the shade.