The "why" is because that is the Proseries solution to dealing with new forms.  This is why 7203 had to be attached as PDF for so long and same with 1116(b).  Maybe next year or the year after it will be able to be filed without attachment.  But Proseries doesn't get the same treatment as Lacerte and Proconnect.  So any improvements to functionality happen in those programs long before they happen in ours.