Because they typically have a wood (stick) body framework with metal or fiberglass panels (and silicone) screwed (or stapled) to the wood. When they start leaking (and they will) think "dry rot." Some "sticky" RVs have an aluminum framework, but the panels will still leak.
Ten Forward - 2014 Escape 5.0 TA
Double Yolk - 1988 16' Scamp Deluxe