Car washes are low maintenance, so it's an easy way to make a little cashflow whipe primarily owning the land for landbanking purposes. With the land freed up for something more productive, other businesses will be set up which are higher effort but higher returns.

Basically car washes are the slightly-harder version of land banking, but there is still enough value in the land that if the land was put up for sale, some other business would purchase it and utilise the land better.

If the car wash owner wanted to maximize returns they could set up a more profitable business today, sure. It's just more effort than they can be bothered with.