I'm glad there's no rain in AC.
First of all, let's take the track grip level into consideration. I don't know exactly how the grip level works when it goes from a green track to a rubbered in hotlap track in game, but is it rubbered in on the racing line, or is it rubbered in on the WHOLE track. If the former, are there marbles offline?
So let's say we have a track that has the line rubbered in, no marbles and you have rain; what is the drop in coefficient of grip on the racing line vs the wide line? Because it is almost guaranteed now that the water ON the rubbered line will be the slippier line; but how much is the decrease in grip. Gotta factor that in. How much do they drop the grip vs how much moisture is coming down? Gotta model that in. What happens to the tires as the track goes from wet to damp to semi dry? Gotta model that in.
When you bring rain into the equation, it's not just modelling the water for the eye candy, it's modelling the entire interaction between asphalt, paint (because the kerbs will react diferently with water on the paint vs when it was dry), water on rubbered track and tire-rubber. That's adding 4 things into the equation. Assuming you have a simplistic model of 3 scenarios (33% saturation, 66% saturation, 100% saturation) you're looking at 4^3 64 additional situations into the mix. It's simplified because it'll be more than 4^3, as iin the case of 33% and under, the dry tire scenario will also come into play and between 33% to 66%, the inters alone will be a headache if that tire is offered.
I can see why they wont' do it, because to do it right will be a huge undertaking; and if you want it half assed, there's always project cars.