Historically, it was not possible for the IRS to figure it out without an investigation that costs more than most people pay in taxes. Computers exist now, but

Historically, it was not possible for the IRS to figure it out without an investigation that costs more than most people pay in taxes. Computers exist now, but changing the status quo would be politically challenging, and may still involve a net cost to the budget because the government would be performing a service that was previously the responsibility of taxpayers. However, the experience of other countries suggests that partial automation is well worth the investment, even though full automation is impossible, so the other commenters griping about lobbyists are correct too.