Except it's very useful to be able to import the many many forms for the various sources of income automatically. If you're just a w2 and a 1099-int then yes, obviously it's not hard to file taxes manually.
PDFs aren't exactly used in a standardized way, so trying to make this work for even a fraction of the numerous tax form issuers requires a lot of work, and a lot of ongoing maintenance. You'll need to prioritize the actual tax forms, and annual updates to those first.
Hypothetically, even if an API existed, building the integration in your own app also costs FTE hours that are likely drawn away to higher priority tasks.