Three reasons I have tons of stuff in my house: Husband, Kids and a dead mother.
My husband is a pack rack. He shops half price regularly and buys about 5 books a months - reads maybe one and hates to part with any. He collects movies and video games - lots bought at GameStop cheap, but still take up tons of room. He just this year relinquished his VHS collect - we don't even have a VCR. Most of it is in his study which resembles a college dorm room with TV, game systems & guitar but it does leak out into our dinning room much to my chagrin.
Kids - at one time my boys (now 5 & 7 yo) had 4 train sets, 2 bins of hot wheels cars & tracks and Legos-lots and lots of legos. I have passed down alot to their younger nephew but some they still use. Plus they have there own libraries of books and games, craft supplies, toys, fish tank and ....
Older son is in scouts so that means sport equipment, camping equipment and just scout stuff (since I am the den leader, it is all stored at my house)
Lastly, my mother passed away in February and guess where all her 'stuff' went? You guessed it - my garage. I joked with my husband that God was trying to tell me something when I had an overwhelming desire to clean out the garage . So much so we could park 2 cars in it before Christmas so we could stuff it full again when she passed. My mom had, no joking, TEN bankers boxes of just pictures which I need to scan when I am able (both timewise & emotionally). My brother, who lives in trailer, wanted some of the furniture - still waiting for him to take it. And my hubby wants to replace his current bookcases in his study for my moms - but he has to move his stuff first.
Now I am not off the hook (holiday decor for Halloween & Christmas take up quite a bit of room in the attic and bath products tucked in containers under the sink), but I dont' buy books anymore - Kindle & Library are my friends. Don't buy movies or TV shows - Rent on Netflix and let them store them. And my game of choice is World of Warcraft - online. Magazines are thrown out within days and junk mail instantly. All my pics are digital. As for kids school papers - I take pics of those I want to remember and chuck most of the originals, but I do have two boxes of memoribiliia filled to the gills with first outfits home, birthday cards & school pics and original artwork. I also have boxes of my memoriablia - photo albums, yearbooks, scout sash and stuff from the pre-digital age.
I despise clutter, but I am the only one in my family who does. If it was up to me I would chuck half the stuff we own, but as a part of a family, it isn't up to me

I am one of those people who wants nothing on their tables & countertops, but it never works out that way...