Chassis manufacturers could easily put heavier springs on their motor home chassis, however you wouldn't want to ride in a rock. Such heavy springs could also affect the relatively weak frames of a motor home.
Owners expect so much from their vehicles, and suspensions are all just one big compromise. They want chassis' to handle the weight of their coaches, while riding soft. And they want them to handle reasonably well--and stop when they hit the brakes. Air Ride suspensions are a good way to meet these goals for a reasonable price.