historically, iRacing has developed from of the old NASCAR titles. In real life, NASCAR never run in rain, it is way too dangerous in packs on banked turns.
that, I suppose is the reason behind there being no rain at the outset, some years ago.
iRacing is now slowly introducing weather effects like fog / damp and wind, all of which feel more sensible to me than the simulated rain I played in games like race07.
I suppose rF02 is out ahead in this area and iRacing nver like to be second best, so they do not even enter the competition before they are not sure they have nailed it spot on. Just my guess.
It will come just as surely as night/day dynamics will come.