Lionels are caricatures of trains and not scale models, weathering would look out of place. Even MTH's more scale oriented equipment tends to look out of place when weathered....er...at least that's my take.
Good question. I think there's two general types of collectors / enthusiasts. Those that go for a sense of realism (HO, N, etc.) with accurately scaled detailing, and those that enjoy the somewhat-toylike feel and nostalgia of model trains (O, some S). There's exceptions to all scale/types, of course, but in general, I think Shay's reference to "caricatures" is well said.
My take on it is this: look at the age of the item being discussed. There is a tendency, with very old Lionel and others, to restore a battered piece to it's original splendor. This feeling might be characterized by the observation that a 60-year-old locomotive that is still running deserves better than the rust and battering that those years left it. On the other hand, a three-year old boxcar that cost $10 new is a prime candidate for experimentation toward realistic appearance. We age the new and restore the old: it's like imparting maturity to the young and honoring the elders.