Robert:

"In 1968 the Internal Revenue Service published Revenue Ruling 68-57, which states that it is the position of the IRS that a PMM or a PMSI given in good faith to secure a loan for the purchase of real property or goods, has priority over an already recorded Notice of Federal Tax Lien."

The liens may have been seen and not dealt with because of the above. I'm sure the IRS figures they will get their money from the equity when the property sells, so there isn't a reason to force payment before.