How so?
Because the withdrawal of one parent as teacher from the workforce would cost that income to the family and the economy. Only low income parents withdrawal would not be very costly. The loss increases as the parent's income increases.
Example: if a parent making $50,000 stops working to teach then the opportunity cost of educating one child is $50,000, two-$25,000, three- 16,667, four- 12,500. Then one must add the actual outlays to arrive at a per student cost. These costs will be much higher than all but the most pricey of private educations.
This is not a reason not to homeschool but all costs must be considered.
There is no free lunch.