Why is this? It has something to do with the fact that public schools are part of our civic religion, the primary evidence that people cite to show that local government serves us. And there is a psychological element. Most of us turn our kids over to them, so surely they must have our best interest at heart!
But do they? Murray N. Rothbard's Education: Free and Compulsory explains that the true origin and purpose of public education is not so much education as we think of it, but indoctrination in the civic religion. This explains why the civic elite is so suspicious of homeschooling and private schooling: it's not fear of low test scores that is driving this, but the worry that these kids aren't learning the values that the state considers important.
But to blast public schools is not the purpose of this article. There are decent public schools and terrible ones, so there is no use generalizing. Nor is there a need to trot out data on test scores.