BA did do them on demand with a select number of items distributed from a basket a while ago circa 2010/11 but feedback from customers was that they were not satisfied with it as they largely perceived it as a cost cutting only exercise (even though it was partly a cost cutting exercise ) and did not consider the waste impact. Likewise with the Elemis shoe bag- it was a practical design and could be used as shoe bag/ cable bag etc but was not luxurious enough. So I think to answer the question - the wash bag adds to the brand rather than being something which is practical or missed.

Washbags left behind , at least in pre covid times, were collected in after passengers had disembarked so they could be recycled.