Dear Jilly -
I imagine part of it is that many of our parents grew up with a lack of things. So when they could finally afford food and clothes and so on they developed a "clean your plate! The food is precious!" mentality. They developed a "hang onto every clothing item! You never know when you might need it!" So the hardships they endured caused them to cling to things and want to be sure to have things.
Coupled with that, marketers now get master's degrees in specialized psychological ways to inflict wants on the human brain. They literally have it down to a precise science. They know the exact colors, exact sounds, exact shapes to trigger a "need" response in a human brain. Since us consumers don't get to take master's degrees in how to resist this onslaught, we are fairly weak in defending ourselves.