User blog comment:Marijuanna/Rare Zanpakutou/@comment-1393830-20130117171523/@comment-4800978-20130119004124

The difference between Ichigo's creation and the average Shinigami is (as I see it) Ichigo possessed enough power at the time to create his sword from essentially nothing.

The average Shinigami however seems to be... well rather powerless. Just souls who are given swords. But over time they grow in strength and eventually can form their own unique weapon.