Its just not only the armour you have to pass through.
For example a FW190a8 hit from rear with a bullet would have to go through the metal skin at a bad angle, the aft fuel tank rear side, the fuel ( if filled wth petrol it REALLY slows down the bullet) then the aft tank front side,then you have the pilot armour , and the pilot clothes and then the pilot.
Did I miss any parts ?
Seeing a wirbewindl that is stationary , flying at 200mph+ is Very hard, if its not sitting on plain open ground UN-camouflaged. Also you do not only need to spot it , you have to put angle with your guns on it , and hit a 2*1.5 meter turret. Also you need some bullets , killing one crew member does only stop them from shooting for a couple of seconds. Our stupid GV icons makes the spotting very easy compared with real life.
When I was travelling in my tank over a snowy road, an attack pilot did not see me , flying right ontop of me, nor the rest of my platoon,according to that pilot seeing small ground targets was very hard. ( this was in the 80's)