Ivan, of all aircraft you have picked one that is notoriously dirty most of its time, you should see the US.Navy counterpart.
p.s. - acc to the leading edge decals I am using on the starfighter, canadians had the practice of washing their starfighers with rags soaked in paint thinner, hence the weathered look on their camouflaged starfighters. Hope they are not treating their hornet the same way
p.s II. - that is not dirt as such, it is mostly paint weathering/fading,paint patchup, hydraulic leaks, oil stains etc.. not mud and so. You can have mud on WWII and helicopters, but modern aircraft operate mainly from paved runways. A big external difference between modern aircraft and helicopter can be seen by simply examining the skin, aircraft have flush riveting(to reduce drag) and helicopters have 'bulged' rivet heads, were drag in their case is not so much important but these also will serve for more dirt to collect around them! A reason behind why modern wheels are painted white is that oil leaks could be more easily spotted. Helicopters operate in a different enviroment and get more weathered and grimy true, in fact these are the only flying things that might get closer and realistically weathered closer to AFVs.
p.s III - see how clean and tidy the ejection seat looks along with the seat belts, helmet, flying gear, HUD, etc..