Because the IRS isn't allowed to tell you what you owe. You're supposed to calculate that yourself... (And it's so damn complicated that it's worth paying someone to do it, unless you were the kid who did the math textbook for fun.)
Eventually, the IRS calculates what they think you owe, and if you underpaid, they send you a bill. You can respond explaining why they are wrong, or pay. The interest is so low that, as long as you don't goof up every year, you're better off risking underpayment than overpayment.
Honestly, the system would work better if the tax prep software could see all the information the IRS has, because then it would be easier to correct their mistakes and harder to make mistakes.