My FIL. He lives in the spacious family home where DH and his siblings grew up, but it now looks like a cross between a haunted house and Steptoe’s junk yard. MIL left him 20+ years ago, before I met DH, when she couldn’t stop the clutter/him getting worse. We haven’t been allowed inside the house for 10 years and it was pretty awful then - carpet in downstairs loo soaked with urine (FIL’s aim is poor 🤢 ), piles and piles of dusty old newspapers and every kiddy handicraft and drawing displayed. DH is 50 and his siblings not much younger.
As someone said up-thread, it’s less about FIL accumulating new stuff, although he’s a sucker for supermarket own brand BOGOF bargains, and more about never throwing anything away. Jars of rusty screws, 10 years out of date condiments, all his children’s clothes.
FIL is well off. He came from poverty but is self made. Very comfortable, but an utter miser, which leads to strange behaviours like buying a brand new car every few years, but not selling the old one. He keeps them “for spare parts”, so the driveway is full of rusting cars on flat tyres going back to 1980’s. He won’t pay a garage to fix his car (“a rip off”) and tries to do it himself using windscreen wiper blades from an old Talbot Samba and a random tyre on his new Passat 🙄