When people move, lots of things that are still perfectly good to use get thrown out. For example, because it doesn't make sense to ship leftover baking supplies, they get tossed. Even if you wanted to give them away, no one wants them once they've been opened. Aside from sanitary concerns, some items aren't valuable enough to dedicate time to selling or donating (e.g. office supplies); others get abandoned if they're not sold in time (e.g. furniture). All of this is in addition to the requisite waste from boxes and other packing materials.
But, we don't need to accept this as given. Furnished apartments in communities with shared supplies solve a lot of these problems. Since tenants own less stuff, they're able to avoid going through this fire sale process when they move out, which seems better for everyone involved.