For rain they have to bake the lighting and cubemapped reflections for each time setting they make for the wet conditions. It just takes too much time.
I do get the fact that the lighting is baked however that doesnt explain why they dont have any races that are SOLELY at night or rain.
Lightmap and Light Probe baking is more or less automated once they're mapped out and placed*. It's likely that >90% of the probe positions and lightmap UVs made for daytime conditions would be suitable for nighttime capture. They'd just leave some threadripper monster build machine on over the weekend.
*Obviously some things like the sharper road reflection systems would need to be verified / tweaked, but again that's a one-time thing that should then work for all subsequent rain conditions.
As for why you can't just blend between 2 sets of lighting? Why can't you have something halfway between those?
Think: the equation for that is (Lighting A / 2) + (Lighting B / 2).
Notice that you'd have to have both sets of lighting data in memory?
Think: remember how the 24h dynamic lighting variants of tracks in GT6 seemed to have lower-quality lighting than the fixed-time tracks....Now you know why!
Maybe it has to do with balancing for Group 4 and Group 1 races? For those classes a multitude of powertrain choices are available, and under rainy conditions, the race will obviously be skewed in favor of the 4WD and FWD cars. Regarding night races, it may have to do with the FIA as well; all GT4 races run at day.
Very good point! In Gr3 there's only a few outliers such as the Alfa and old GT1 cars, whereas with Gr4 there's the Alfa and GT86 (super-light MR/FR), all FWD cars (FWD), as well as the Megane Trophy (super-low polar moment, short wheelbase), and several larger FR muscle cars, usually with wheelbases that differ greatly.
The counter-point to the performance you mention is that MR cars should be able to get better drive out of corners; in the wet race at Red Bull Ring last time, the MR cars were flying out of the first, second, and last corners. In Gr4 though, with straights that long and average corner speeds that low, I can imagine the top speed (least affected on that track by rain in Gr4) of the FWD cars would make them even more unbeatable than normal...
IBM Cell power (PS3) Vs. AMD Jaguar low consumption laptop CPU (known as PS4 bottleneck)
The bottleneck for these conditions is GPU-side. And that'd probably the biggest differentiatior in PR4 vs PS4 Pro; while the Pro might be able to handle it fairly well, the OG ps4 might struggle a bit too much to give a confortable framerate.