It just occurred to me, there may be a reason they don't tell you that I hadn't considered before. If they tell you exactly what income they do know about, then they're implicitly providing information about your income they don't know about. This might make people with harder to trace income less likely to pay taxes on it, as they have some upfront assurance that they won't get in trouble. In some ways it could be seen analogous to a common rule of negotiating, which is to get the other party to say a number first. This allows the IRS to prevent lowballing the tax number, and if the person comes in with a low number, they can still "negotiate" it up.

I don't know, just a theory. I still think the whole complicated process is stupid and they should just give you a number at the end of the year.