The success of the off-site storage business (with garage-sized spaces crammed with stuff people don't even think about) shows that hoarding is widely practiced.
My wife hoards Christmas decorations and stuff, so we've had to rent a small storage space to keep it all.
My mother was a pack-rat; it took us a week to throw out all the junk she accumulated after she passed.
That could be hard --- it depends on the yearly cost of storage --- but I can imagine there are things stored in some of those places that don't amount to what people pay to store them.