why do the three friends not wash their clothes themselves while staying at Streatley ?

The men just made their clothes more dirty and worn when they tried to wash the clothes themselves:

Washing line We stayed two days at Streatley, and got our clothes washed. We had tried washing them ourselves, in the river, under George’s superintendence, and it had been a failure. Indeed, it had been more than a failure, because we were worse off after we had washed our clothes than we were before. Before we had washed them, they had been very, very dirty, it is true; but they were just wearable. After we had washed them—well, the river between Reading and Henley was much cleaner, after we had washed our clothes in it, than it was before. All the dirt contained in the river between Reading and Henley, we collected, during that wash, and worked it into our clothes. ch 17