I think this is largely do to having a progressive and conplicated tax system. If work one job, earn salary, don't receive a variable bonus, and work the whole year at the same rate, it's easy enough to calculate.

But, get a raise, take a pay cut, find a new job, start or stop a second job, or a third, or fourth, get paid hourly, get sick, take time off to care for a sick family member, have a kid, go back to school, graduate, get married, get divorced, get a mortgage, pay down your mortgage, ... There are all sorts of scenarios that could change your tax situation on both the federal and state level, making it difficult to calculate how much you would owe up front.

What it really boils down to is our tax codes suck, and too many rich folk are keen on keeping it that way.