Because it is not applicable / relevant.
A Product Line (generic) is not a real bit of physical inventory - it is a virtual piece of inventory. So it can't be READY or CHECKED_OUT. (it cannot have a status - it is a placeholder for when you know which real inventory will be allocated)
You sell (rent) a Product Line. (perhaps online or via back-office)
You then allocate inventory to that Product Line. (which shops the virtual for a physical)
The inventory you allocate can have a status like READY or CHECKED_OUT etc
See Also