The rebels had tatty ships because they had to maintain ancient machines, the Jawas sold tatty robots to moisture farmers because that is what they could salvage and the townships of Tatooine had speeders with dents in because they were average Joe farmers going about town.
When you are showing a royal flight for example, this is a ceremonial fighter group.
It may be old but because it's ceremonial it looks brand new.
Airforce One looks new because you don't want the leader of the world's superpower stepping off a tatty looking rust burger. It doesn't inspire confidence.
The look should be applied in a logical way not just for the sake of applying it.
Show me the rust, dents and flaky paint on the Imperial ships.