Laundromats take a lot of time, cost money, you have to travel to and from, there might not be one nearby, and I can think of a lot of things I’d rather be doing rather than spending hours in a laundromat!
Why use a laundromat when you can do laundry right where you’re camped? Of course it may not be possible if you’re parked in the middle of the city on a side street, but if you can hang a clothes line, it’s easy as long as the weather is dry enough.
A week’s worth of laundry takes me around 20 minutes from setup to hanging the clothes to dry. In the hot desert southwest with a little breeze my laundry is dry in less than an hour. If it’s cooler or less windy maybe all afternoon. And I don’t have to leave camp, and can do other things.
There are several ways to do it - I use an old-fashioned washboard and the dishwashing tubs with a bar of laundry soap, because the washboard is flat it takes very little storage space in the van. New washboards can be found for less than $20 online. Another good way is a 5-gallon plastic bucket with a toilet plunger and liquid soap (if you already carry a bucket). If you like to blow money on gadgets there are commercially made manual-crank wash tubs, but I don’t think they work as well, and take up space.
To me, doing laundry in camp is a lot more convenient.