This site would be slow, the code base would be unmaintained, it'd get enormous amounts of spam you have to sort through to get some important tax document, and it would be down all the time. Think the line at the post office but for server up-times.
Similarly if the mint maintained a payment processor then they'd just create a legal monopoly (like the USPS did) and ban new processors. Not only would they be worse than VISA and MasterCard, but they'd make paypal and venmo illegal. Don't forget the USPS bans competitors from being cheaper than itself, and this is exactly what would happen if the Mint had its own payment processor.