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

GBAL overview