The average exterior oil paint will get a dusty haze (oxidation) on the surface, making it look unsightly. Flats paints also "burnish",i.e. get shiny marks where something has rubbed against it. Flat paints look flat precisely because the surface is rougher than gloss paints and disperse the light rays, rather than reflect them directly to your eyes. Any rubbing of the surface polishes it. This is why it in inferior for doors and hallways walls and kids bedroom walls.

I would grant that good oil paints of years past are superior in many ways. They are all I used on fine woodwork. I hate acrylic paints on doors and fine woodwork! Unless sprayed, they don't flow out as well and they retain that gummy feeling which makes them want to stick where surfaces meet.

By the way, Frau Ordjen makes me wash the front door all the time! That is what I get for having married an old world German!