That's because zero times anything of any magnitude equals zero. Considerable evidence suggests a multiplicative relationship. Children who tend to do well in school generally come from two-parent families. Their parents ensure that homework is done and educational material is in the home. They're lectured about school behavior and punished when necessary.
Parents make sure they attend school every day and on time.
Parents respond to school notices and grades. And, they make sure their kids go to bed at a reasonable hour. A good home environment is vital to getting a good education. The measures that parents take to create a good home environment cannot be done by government or educators. If what only parents can do is not done, education results will always be disappointing.