Because Microsoft are moving away from selling boxed editions of Office.
These terms btw are not new. They were there in Office 2010 for OEM editions of Office:
15. TRANSFER TO A THIRD PARTY. You may transfer the software directly to a third party only with the licensed device, the Certificate of Authenticity label, and this agreement. Before the transfer, that party must agree that this agreement applies to the transfer and use of the software. You may not retain any copies.