A logical enough question. But that's the problem, it's too logical. Or, rather, the logic immanent in the question is inappropriate for understanding why many languages are equipped with a 'vestigal' middle voice. One can see what I mean if we consider what is meant by 'property'. In the guise of the term 'attribute', this is normally opposed to substance or substrate. The chap who introduced this distinction, Aristotle, wanted activities like washing to belong to the attribute side of the opposition. He specifically did not want the substrate to move. This was a revolution in ideas on physics (that lead to a revolution in ideas on the functions of the verb) for, formerly, the substrate was believed to move and this movement was believed to be the cause of some of the characteristics of the things possessing self-moving substrates. So when Aristotle banished self moving substrates from (his) physics, anything affecting the substrate had to be explained (actively or passively) relative to things outside it. This revolution in ideas on physics lead to a revolution in ideas on the function of the verbal voice. The grammarians believed that the verbal voice should express activity or passivity or, in the case of clearly intransitive verbs, pronomial reflexivity. They must never express a self-affectedness attibutable to its substrate, which is precisely what the pre-grammatical middle existed to express
The importance of saying this isn't to claim that by anyone's logic it is the substrate of the jeans which acts on them in such a way that they get cleaned easily. It's only to say that verbs expressing agentless self-affectness are relics of a linguistic heritage that accepted this as normal lost in an (re)organisation of language that doesn't accept that this kind of think can happen.