As part of the public/private partnership that started e-filing in the early 2000s, one stipulation is that the government couldn't compete.

There was a lot of noise made last year when a law almost made that permanent.

I'm not sure if the API you're talking about would apply, but I'm pretty sure it would and passing a law would be difficult.

https://www.propublica.org/article/congress-is-about-to-ban-...