It's not just oil and diesel, a lot of it is due to tyre compound deposits which stick to the road's surfaces very well. Flash floods aren't actually that bad, most people are pretty cautious through flood water. Plus, the large volume of water does help to flush away surface contamination pretty effectively.
It's the light dusting of rain after a dry spell which really catches drivers unawares. I find the autumnal leaves and grime just as bad.