Tax purists would argue that the value of the homemakers' hard work--and the intrafamily benefits they presumably receive in return for it--should, in fact, be treated as income and taxed, just like the wages paid to outside service providers such as baby sitters and housekeepers.
People who are strict about tax rules think the work done by homemakers and the benefits they get from it should be treated as income and taxed just like other service providers that get paid for what they do.