Throwing stuff out is hard to do if you are poor or were raised poor. Everything you save now does not have to be bought later. There is a potential use for anything and everything, so you might as well keep it. Granted, at a certain point the cost of the storage facility exceeds the amount saved, but the mindset is still there.
Even now, as well off as I am, I tend to collect things (and by extension, clutter) because I can see the potential need for something in the future. I've been trying to overcome this mindset and slim down my possessions to the things I need and the few sentimental items I have, but it's not easy at all. If someone else came into my house and started trying to clean up, I'd probably lose my mind (admittedly for a few unrelated reasons as well, but largely because of this mindset)
Might be worth watching an episode of the show to better understand what Kondo's process is. She doesn't clean or work some one off magic. She teaches 5(?) core lessons to help maintain tidiness. She doesn't excavate their piles of stuff for them.
Even now, as well off as I am, I tend to collect things (and by extension, clutter) because I can see the potential need for something in the future. I've been trying to overcome this mindset and slim down my possessions to the things I need and the few sentimental items I have, but it's not easy at all. If someone else came into my house and started trying to clean up, I'd probably lose my mind (admittedly for a few unrelated reasons as well, but largely because of this mindset)