I don't claim to know or understand, but I can venture a guess: as far as I can tell, America doesn't really have a good digital identity solution. In fact, given the frequent online discussions about whether it should be allowed to vote without physical identification in America, I'm given to believe that you don't have a good personal identity system at all!

The above combined with the to me foreign notion that tax filings could be considered private(in Sweden, everyone's income and their complete tax filings are public information) probably makes it problematic for them to just send it out willy nilly. What if they sent you the wrong filings?

Based on a very widespread stereotype about Americans, I imagine the irs would get sued.

But, as previously stated, I've only been in America long enough to switch planes, so my guess is likely to be inaccurate!