The need to wash may be purely psychological.
some people who want the fabric of their jeans to age and wear in a specific way, don't wash them, but instead manage odour causing bacteria by periodically freezing their jeans.
Well sure, smelling comes from bacteria, but what if I get dirt on it? Will it resist that? Sure the water repellant stuff is helpful, but I find it hard to believe it'd keep it perfectly pristine.
My question is the opposite. Why would I wash it if it's self-cleaning? Smelling comes from sweat bacteria; if sweat doesn't stick to the material then it should never get smelly. Obviously I misunderstand key aspects of this whole thing.