This aircraft probably did not fly for a considrable amount of time
I should say that most aircraft in FS are representations of commercial aircraft. The airliners want to keep their aircraft relative clean. Passengers will undoubtedly be more happy to step in an aircraft that looks clean, sounds reasonable I think.
Anyways, commercial aircraft are far from really clean, if you come close enough. Especially near regulary used inspection panels, outlets and of course the landing gear.
