The threshold rates for a lot of credits, deductions and exemptions are not. Like the Roth IRA is $153k for single, $242k for married. Child care credits have a similar problem if I remember correctly (or they did before my kids grew out of it).
Portland, OR metro gives a $125k threshold to single and only $200k to married filing jointly for supportive housing taxes, which is a 1% tax on income above the threshold.
I have no idea why households that tend to demand less housing (married usually live together) are charged more than singles, who typically demand their own housing at that income level. It makes no sense!
The thought being it's okay to steal from married households because they intentionally set themselves up for lower per partner overhead? Why don't we charge single taxpayers with roommates on the same schedule?
By living in a single house, a married couple demands less housing, which reduces the cost of housing against a fixed supply. Why are we taxing the people making housing cheaper under the guise of "affordable housing"? Shouldn't we treat affordable housing taxes like the carpool lane and reduce the burden of those that are reducing the burden on the constrained supply?
I did a draft of our taxes this week and it was almost exactly the same amount filing married vs separately. Where is the big benefit for filing jointly? I guess if you claim dependents?
MFS vs MFJ is a whole different thing from MFJ vs 2x Single.
While there's numerous places where MFJ < 2x Single due to various marriage penalties, and even more numerous places where 2x Single < MFJ due to the shared tax bracket space below the 37% bracket....
there are very very few places where MFS > MFJ. They exist, but you have to be in particular situations like one person having disproportionately more debt and on an income based repayment plan, or similar.
There are of course many situations where 2x Single = MFJ = MFS if it's just two similar income W2 employees with no edge cases going on.
(Once you're married, filing 2x Single is obviously not an option)
If one spouse makes significant income and the other does not, it can help drop the high-earner into a lower bracket overall. Not a huge boon, but every penny counts in our household.