Even if the IRS doesn't know, for most people the difference between the right answer and the answer the IRS has despite missing some data isn't enough for either to worry about. My bank reports my stock trades, my company reports income. My state already collects mortgage information so it wouldn't be hard to send that on. Sure I could build a widget in my garage and not report taxes, but either I'm doing so little of this that it not worth the IRS's time, or I really need to become a real business and get an accountant to report this while dealing with the other complexities of finance.