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Why do Americans have to file taxes, even if their only source of income is from a regular job?
One of the reasons, apart from historical, is that different people have different tax liabilities which the employer may not be aware of. For example, in the US we don't pay taxes in source on investment income, and there are many credits and deductions that we can't take. So if I have a child and some interest income from my savings account - employer's withholding will not match my actual tax liability. There are credits for children, additional taxes for the interest, and the actual tax brackets vary based on my marital status and filing options I chose. So even the same family of two people married will pay different amounts in taxes if they chose to file separate tax returns for each, than if they chose to file jointly on one tax return. For anyone who've lived anywhere else, like you and me, this system is ridiculously complex and inefficient, but for Americans - that works. Mainly for the reason of not knowing anything better, and more importantly - not wanting to know.