just4mrg,
Well, a couple of reasons. The first is probably because your house isn't bouncing and twisting and turning down the roadway. This extra motion puts additional stretching and strain on connections and sealants.
Also, the construction of most RV's isn't as robust as your home, and a leak, even a small leak left unchecked, can quickly cause very expensive damage to the roof, walls and floors of the RV.
Third, I'm sure it's a CYA thing for the warranties. "Oh, you didn't check your roof seals every month? Sorry, we can't pay this claim.". Probably an exaggeration, but that's the idea.
Now, I replace any deteriorating seals on the roof with
Eternabond Tape and that seems to solve the problem permanently. Great stuff!
~Rick