Why doesn't Self-Service Receiving have the Add to Receipt button like warehouse receiving?

The Add to Receipts functionality is only available within warehouse receiving.

A warehouse manager or receiving agent has the information to identify how receipts should be grouped. In fact, a warehouse user can even enter a receipt number manually. In warehouse receiving, the user can create a receipt for purchase orders that belong to a single supplier at a time. Also, receipts can be created for only one inventory organization. Because of that, the warehouse user needs the ability to access previously created receipts and add to them.

An employee who accesses Self-Service Receiving doesn't have the necessary information regarding how receipts are grouped. More importantly, a requester can receive purchase orders belonging to different suppliers and destined for different inventory organizations. That's why Self-Service Receiving organizes the lines being received and creates the receipts based on different grouping criteria. Self-Service Receiving always creates new receipts.