Return to receiving and Manage shipment lines

Summary:

Setup:

In Receiving parameters, we have enabled "Print shipping documents for returns to suppliers"

User action:
Perform PO Return to Receiving transaction.
In Review Completed Transactions ,inventory is reduced .
In Manage Item Quantities, the ON hand qty shows it is still in Receiving area.

The system didnot create Shipment lines- (It would make sense to generate shipment lines), so that warehouse team can pack it and then perform shipment confirmation when the material physically goes out of the warehouse.

Question:
Why is the system not generating shipment lines when we perform Return to Receiving area?

Imagine if we have 3PL or our own WMS involved in managing the Return to Supplier process ,it will not marry up end-end.