Rain = simulation though. It happens in racing all the time, it happens when driving. Cars & rain go hand in hand. I understand why they perhaps didn't do it last-gen, but this gen, it has to happen, IMO. There really isn't a valid reason not to do it.
They would have to really dumb down the tire physics to do it, making the game much less of a sim (see FH2).
The tire physics made a huge jump this gen. This is the first version where I don't need telemetry to tell me what the car is doing, I just feel it. You can actually drift now too without having to create some weird tuning setup. You're really asking a lot of a $60 console game to take that and add in all sorts of different wet conditions realistically.
There are so many times I go into a corner and the back end gets a little loose. I scrub up the track a bit and lose speed, but never really lose total control. I've never felt that in a racing game before. In past games, getting loose usually meant you were done. In this game, it means that you are losing speed and sliding around, but as long as you drive properly you can still keep the car under control.
What other games do is simply say in the rain you get some % less grip overall and be done with it.