Probably because that is what real planes look like and simming by definition means replication of the real aircraft.
Obviously many of these aircraft haven’t been manufactured for years so if you want a realistic interior then you get the wear and tear the aircraft had.
Some companies make custom textures so you can have a clean version but they almost never look as realistic (often they are pretty poor, it takes real art skills and lots of time to do it well) as textures produced from a real aircraft.