Oil, coolant and fuel drip or gush from damaged engines and ruptured tanks, splash onto the road, and pool into puddles that make it genuinely slippery. The more that spills the bigger and deeper the puddles get, and the more grip you lose driving through them. Tires pick the fluid up, smear it down the road and slowly scrub clean again. Coolant and fuel dry up after a while, oil stays until you clear it.