It looks like a huge chore to wash dreadlocks. They must be undone, washed and redone. Must take hours to do. They must not get washed often.
I have never seen .. either personally or in digital/photographic/television displays .. dreadlocks that looked clean. So I would have to say that I agree with your final statement ...
Apparently some people have nothing better to do with their time than carry around a headful of dirty braids that make them look like Medusa.