I think this explains a lot of it. Another part is that people don't think about the costs of owning stuff: it occupies your space, you have to keep it organized, and you have to move it around whenever you move.

These costs are easy to ignore, because they aren't in mind when you're thinking about buying a specific thing. The mentally-available facts are "what will I get by using this?" versus "how much money does this cost?" Similarly, when you're looking for stuff to get rid of, it's hard to bring those costs back into light, because they're so general to everything you own

I don't have lots of stuff, and I'm pretty willing to get rid of stuff or give stuff away. I think this is largely because I highly value my space, my attention, and my time, and I've practiced being sensitive to those values when I'm making decisions about stuff.