I think USA has the additional complication of having both State and Federal income taxes. There also seem to be a lot of things that can be deductable there that don’t apply here - like mortgage payments on your own home and the value of donations given to charity (not just money, but stuff like old clothes you drop off at the op shop!). Plus they have the option to file a joint tax return as a couple instead of being treated as two individuals, which is beneficial in some circumstances but not others.