Gee, you're the first person ever to raise that point
If you're serious about taxes, the justification traditionally has been that it promotes families, which are considered a strength for the nation. I am not agreeing or disagreeing; just explaining the reason for it. And sometimes, in some states, being married can cost more tax-wise than being single. I don't know what the federal tables look like now because I've filed jointly for so long. But Iowa gives married people the option of filing as individuals in order to get a lower tax bill.