Disassembled Furniture - Why are the screws never there?

When you buy a unit that contains disassembled furniture, why is it that the screws are never there? Surely the previous owner could put them all in a zip lock bag and leave them in an obvious place for the person who's going to buy their storage unit. 

lol was laughing at this post. i just moved this past weekend myself the move made all the more difficult not by my stuff but all the crap ive aquired at auctions. i cant find the screws or dowel pieces to my bed.