because the federal government tells us to. For our family, it's not about the money. Our tax penalty as single filers is negligible, and our finances aren't complicated enough (aka rich enough) to make separate filing burdensome, but it still irks me. I consider myself an honest person, and that makes it really hard to check the box labelled Single, when I know that Badger and I have a marriage certificate.
It didn't help when I learned that in states with common law marriage, straight couples who publicly hold themselves out as married can legally file both state and federal as a married couple even if they have not been formally married.