The program has to track the items by item number, and the item numbers must be consistent across all stores. The only way to ensure that this is done is by having the central store be the only one to create new items. If multiple stores created new items, you would have situations where two different items created at two different stores had the same item number, or two different stores created a new item in the system for the same product, but created it in a different order, so the same product would have two different item numbers.

If the item numbers are not consistent across all stores, there is no way for the central store to report on the items correctly, or track the transfers between stores, or handle any of the other item related multi-store operations