I think they already do in a sense; that’s the standard deduction. If you want to maximize your returns you might be better off itemizing though, and that option is what makes everything complicated (I suspect they’d have a hard time sending you a bill for everything you itemize… I don’t know that they really know everything you could itemize; I think that really only comes up when it’s suspicious/you get audited).
i.e., if they did that, you’d basically get fewer options, and maybe less money back(?)