Uh, why would they be? The IRS is the government and people sue them all the time and often win.
In the United States, the federal government has sovereign immunity and may not be sued unless it has waived its immunity or consented to suit. The United States has waived sovereign immunity to a limited extent, mainly through the Federal Tort Claims Act, which waives the immunity if a tortious act of a federal employee causes damage, and the Tucker Act, which waives the immunity over claims arising out of contracts to which the federal government is a party.