Generally, income class and intelligence are highly correlated. So your answer is that they are stupid.

Because they don't ever want to be without something they might need in the future. I have a hoarder family member and this is their reasoning. They also enjoy having something to give to people if they need it.

They can't pass up "free stuff" (junk) and having it makes them feel like they own a lot of things...

They probably don't want to waste things they later turn out to need. If they can't easily just go buy whatever they need, that junk may be all they've got. Also their lives are probably a mess so they aren't in a mental space to figure out what things they can get rid of.

Mental illness and/or drugs.

I’m gonna use all that stuff someday.

You just can't beat that " just won the lottery" feeling of finally mowing the yard and finding that old rambler body you traded a half case of Busch lite and a quarter of meth for 3 years ago but then forgot about after not sleeping for a week and a half due to a three 8-ball bender.

The same reason they're poor

Hoarding useless shit and turning their yard into a landfill makes them poor