I have to say though, that I dont think all hoarders are the same. Yes, some have mental illness. Some don't. Some use it as a way to have control over their lives, as a way to have control over others, and some just plain don't ever spend the time to go through their stuff.
My mother is a hoarder, and I grew up in a house that looked like something you'd see on the hoarding television show. Boxes above my head, 8-10 inch paths all over the house, collections of styrofoam cups that came from fast food restaurants, and just general filth. We cleaned it up over and over and over again- got the house back to clean slate. And it always happened again. Along will all the other mental illnesses my mother has, OCD the primary culprit. She won't throw away anything that has her name on it until it's shredded, but she lets that stuff pile up for months. She won't throw away recycling until it's washed out and not sticky, so there are piles of coke cans. She won't throw away styrofoam because it's bad for the environment, so in order to justify her purchase, she has to reuse the cups. Her house is cluttered because she has projects lined up for them, because she thinks one day we will want them, or because she needs to "organize" it into wherever it "goes". She used to make us sit around and go through the stuff with her to help figure out what to throw away. Hours and hours of paying attention to her and her stuff. At one point in my life, we were paying for three separate storage units that held her stuff. It cost more than her mortgage did. She has never, ever admitted that it's a problem- even when she has been told that her grandchildren cant come into her house because of it's state. Even when CPS took us away and made her clean the house up. It's always the same thing- she wouldnt be in this situation if her kids and husband would "do their part" and help out around the house. It's always someone else's fault. She's even had state mandated therapy for it.
My MIL had one of those houses that was clean and full of antiques when I started dating DH 10 years ago. She had a 10 year old daughter at home, worked full time, and was the president of her college alumni club. Over the years, I have watched her house fall into shambles. When SIL got to high school, she was hanging out with friends a lot, MIL still worked full time, and to occupy her time she joined clubs and committees so that she wasnt home alone. This carried on over to when SIL went to college. She is never home, and when she is she just heats something up in the microwave and watches tv and goes to bed. She doesnt spend time at her house- so she often misses trash day, she doesnt go through stuff and make bags to take to goodwill. When things happen like her printer breaks, she buys a new one, and just moves the old one over to the side. It's not that she wants to keep it, its just that she doesnt "have time" to do anything about it. She is still in the old school mindset of everything she sees on the internet she needs to print out. So her house is covered with recipes, geneology information, and papers from all of her clubs and committees. She doesnt want to keep all of it, but she doesnt really want to sit around by herself and go through it to throw it away. Her mail slot has a pile of mail that is 2.5 feet deep at the base of it. So, needless to say, there is no where in her house available to hang out. We dont do holidays there, we dont visit there- there is no where to sit. Now, in this situation, it's again, someone else's fault. The reason her house is cluttered up is because DH and SIL won't come help her. They are "never home" (yk, like adult children arent when they no longer live at home) and she doesnt feel it's her responsibilty to clean up her own house. We've cleaned it several times, but she wont let anyone throw anything away until she has reviewed it- and that takes a long time. DH is paying for a pod to get some of the stuff out of her house so that other stuff can be sorted, but she is pissy about him paying for a pod because she thinks that he needs to come over and help her sort through everything. You know, with his full time job, his hugely pregnant wife, and his two year old. Cause he has nothing else to do.
IMO, these are three very, very different types of hoarding. My mom is mentally ill. My MIL is depressed and never home. What I do see with both of them, is a way for two mothers to control their children's time. Both of them want constant, one on one attention from someone at all times and people feeling forced to help is one way that they can get it. So I know there is a lot of judgment and resentment that goes a long with this, but IME the women who hoard in my life are very controlling and they have no problem with the fact that their problem manipulates someone else's time to an extreme extent. They are both people who feel like their children "owe" them something.