Hoarders are so disturbing because they can harm so many more animals than just a single scum bag who neglects or abuses their pet.
You are correct in that it is an illness and I also wonder why family or at the very least neighbors don't get involved and speak out for the helpless animals who can't speak for themselves.
My mother worked on a huge horse seizure several months ago and those animals could be seen from the road looking like bags of bones. Even someone with no horse savvy could tell they were emaciated! Why didn't someone make a phone call or ten if needed before it got that bad?