Why? Well, because the builtin scripting methods and APIs in Adobe
Acrobat/Reader only handle a singe file. Here's the documentation of
Acrobats JavaScript
API:https://wwwimages2.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/AcrobatDC_js_api_reference.pdf
Here's the LiveCycle scripting reference:
https://help.adobe.com/en_US/livecycle/8.2/LiveCycle_Designer_Scripting_Basics.pdf
See table on page 85. It lists, what scripting methods from Acrobat JS
API are used.