Yeah that could be a concern depending where you live. Where I live, water is cheap and abundant. And I've never measured, but I would guess I use maybe 10 gallons to wash a car. 2 gallons in the bucket, and just a light wetting followed by soapy sponge and then a rinse, using a nozzle to create a high velocity spray. The water doesn't run at all unless I'm spraying it on the car.

Water is cheap and abundant where I live, too. If we have a problem with water in any normal year, it is usually that we have to deal with a surplus of it. (We put drainage tiles under our farm fields to get rid of excess water and generally don't even bother with irrigation systems at all.)

But car washes are interesting systems and one of the things that makes them interesting is that the water they use is (often) recycled. The same water gets reclaimed and re-used to wash cars over and over again.

I didn't look very deep at all, but one car wash chain suggests[1] a reclamation rate of 70%. Not perfect, but way better than 0.

[1]: https://www.tsunamiexpress.com/our-washes/