The obligation (as laid out in many state constitutions and as part of the generally agreed on social compact that has existed for over a century) is to provide a uniform system of common public schools within a state.Because you and others have it wrong. You aren't subsidizing 'schools', you are subsidizing 'children's education'.
The obligation is to all of our kids. Actual people. Not to a building, institution or system.
Vouchers subvert that.