[notanormalnerd]

It is mostl due to the oil and other hazardous materials potentially going into the ground or the city sewer.

They can't or won't clean that and it is contaminating in even small amounts. E.g. one drop of oil contaminates 500l of water.

At least for Germany.

[michaelt]

Doesn't any oil on the road etc end up in the sewers next time it rains anyway?

[throwup238]

Yes but they still degrade relatively quickly when managed correctly so a burst of pollutants whenever it rains is better than a constant low level exposure from people washing their cars all the time.

Regions where it rains frequently like the PNW have rain gardens, vegetated swales, catch basins/filters, and other mitigation strategies all over the place whereas e.g. California might just have them throughout the drainage system like at the end of the LA river.