A lot of it has to do with taxes. These carwashes are basically just real-estate parking. The formula is simple, buy a plot of land in an area you think will be high demand, plop a carwash on it to reduce taxes for owning that land (maybe pull in a few bucks here and there), wait for the land value to appreciate significantly, sell it for a profit to future development (or another person who thinks the land will further appreciate).

End result loads of carwashes doing nothing. Pretty sure it was the same reason mattress stores were everywhere at one point. You just need like 1 person to operate the store so paying their salary is nothing compared to the future appreciation of the land value.