Answer: Because our nation and freedom as we know it will cease to exist.
Our government's socialist-entitlement and single-payer K-12 schools are our nation's and freedom's MOST serious threat....yet...Day after day Christians and conservatives put their kids under the care of freedom's enemy.
You are assuming that since leftwing indoctrination is the purpose of the public schools, and it is, then the children in them will be indoctrinated.
But that is not necessarily true. Children understand when they are lied to and if the parents give their children support at home, they will grow up immune to the indoctrination. It is however important to discuss issues of religion and social policy with your children and give them intellectual tools to combat the leftwing propaganda they receive at the school.