Probably nostalgia. Lots of games I will never run (or run again). Nowadays, I mostly buy PDFs.
Well, also, I bought some 5e books, you cannot buy those in PDFs, but I've stopped buying 5e stuff a while ago.
I definitely wouldn't rent space for my books (I don't need to, I have fewer than 100 books), but I might be a PDF hoarder (I have about 100,000 files in my RPG folder, but that includes stock art, character sheets, redundant copies, zipped file with multiple books, etc. All legal, most bough in sales or bundles).