I think much of it is a combination of jet-setting affluence, and of international students, who might not be affluent, but who can't take it with them, and are too busy to sell it.
The sad thing to a curb-shopper is the knowledge that most on-campus discarded stuff never makes it to the curb, other than "Allston Christmas".
The other day, when there were a lot moveouts happening (when you can actually "pahk yah cah in Hahvahd Yahd") someone had set up some large donation bins, so hopefully the things are going to someone who will use it, rather than to a landfill. (The city now also runs year-round clothing donation receptacles.)
Earlier this month, I helped this person who needed help getting a nice piece of furniture from the curb into their car. She capped off gushing about the nice find, with "Harvard students!"
What restores a little of your faith in humanity is when people will go out of their way to donate, or to put something carefully on the curb with a sign, when it would be easier just to toss it into the dumpster.