I can read tax law, but the problem is you get a month between when the IRS release the rules and when you have to be done - and correct. The big companies put in a lot of work to enter all the rules in the month, and then even more work to validate they are correct. That is boring work that needs to be done every year. If you get something wrong it is the users who find out when the IRS rejects their forms. While it isn't a big deal for that to happen to one person, it is still annoying, and if there are lots of users who hit the same typo that is a lot of annoyed users.

The FSF just doesn't have the ability to do a good job with tax software.