Because most use marginal components for cost purposes, and they sit for 50 weeks a year with no exercise or maintenance and then get pulled at faster than design speeds with heavier than design loads.
Happens to commercial trailers too, most wheel, tire, bearing, spring failures are on the trailers, not the tractors. For many drivers, the tractor is a responsibility and livelihood, it gets taken care of. The trailer is something to be hooked up and taken to whete it is dropped off, and you are kind of hoping that the operating company is taking care of it.
Since towable RVs are usually owned by the person towing them, they should be better cared for, many are not. Many RVers don't have the knowledge of what is needed and how to do it.