becasue how are publishers meant to know when a parcel arrives at a store?
Gee, if only we had some way of guaranteeing when shipped packages would be delivered. And maybe some manner to track those parcels as well. Oh, and it'd be really cool if all of that was tied into some global information network.
Oh, and lookee... I just happen to work on software that does exactly that kind of thing (No, not for UPS/FedEx/USPS -- for the trucking, shipping, and courier industry). Funny that.
Movies (both in the theaters and VHS/DVD), books, and all other retail sales seem to have no problems with setting a sale date for products. The game industry is the only one that doesn't do so. It's really quite absurd.
Heck, most of the entertainment industries even have a standardized date for new releases. New albums come out on Tuesdays, with rare exceptions. New VHS/DVD titles are also Tuesday. New movies debut at the box office on Wed or Friday. New games, however, come out whenever the publisher can actually ship a box out of the door, which causes headaches for the entire distribution chain. Give it another 10-20 years and I'll bet we see some standardization on release dates though, presuming we don't move to an entirely electronic distribution methodology.