if i'm understand this last statement, you are saying that if one person is in a C172P and it has everything implemented and someone else is at the same place at the same time but in a craft that has few or none of these features implemented, the other person won't see things like rain splashed on the water puddles, or even rain on the windows?
The rendering engine doesn't texture airplanes either - that's a job done by the aircraft maintainer because the rendering engine can't know what texture the airplane is supposed to have.
Likewise, the rendering engine can't know that a surface is to represent glass unless you tell it via an effect declaration. It can't know how the airstream goes across the glass unless you tell it via a splash vector. It can't know what the surface structure of the airplane is (in terms of bolts, rough patches,...) unless you tell it via a normal map.
So in general the rendering engine will do what you allow it to (via the rendering controls and the quality selection slider), connected with a logical AND with what is able to do because it has the information.
For instance, if you declare a surface as glass and it is raining, the renderer knows that it should paint a rain pattern and does that, but if you do not specify a splash vector, it doesn't know how the airstream goes across it and never changes the rain pattern as the plane accelerates down the runway. You will always see rain splashes on the runway if the quality setting is high enough, because the runway never changes with the plane you choose and the rendering engine always has the information of the layout.
But we can't pre-suppose that all transparent surfaces represent glass - in many cases (think grilles painted on a texture) they represent air, or semi-transparent surfaces may represent fog (see typical particle textures, or the whole set of cloud textures),... so the renderer needs that information.
Aircraft effects usually need to be implemented per-airplane because they need airplane-specific information, it's as simple as that. And they're optional because not every maintainer wants to go to the same level of visual detail.
We can't actually encode the fact that it is raining into the scene simulation and let the renderer do all the physics - that'd give you still images at a rate of a frame per minute. We need to be much faster, and so it's a game of visual illusions and pre-computing whatever can be pre-computed.