Federal taxes tend to be even harder than state taxes. There's usually more that goes into whether or not you qualify for exemptions and what sort of deductible you should take.
But the reason it's a mess is really really simple, lobbying. Tax prep companies have lobbied against a government solution for a long time now. Requiring everyone to spend $100, $200, or more per year is their subscription model and they like it that way.
Federal Taxes tend to be easier than state taxes. First of all, there is no nonsense of partial residencies to consider, which gets even more messy when you and your spouse have spent time in two different states.
Additionally some states (I am looking at you CA and NY) are absolutely rabid when it comes to coming after you for what they deem is their share. They will keep hounding you for years after you have moved away from the state. NY in particular enforces a withholding on additional income that's more than the maximum state tax rate, which effectively means that they get millions of dollars of interest-free loans from their taxpayers.
Plus community property laws make calculations even more messy.
But the reason it's a mess is really really simple, lobbying. Tax prep companies have lobbied against a government solution for a long time now. Requiring everyone to spend $100, $200, or more per year is their subscription model and they like it that way.