Vastly depends on the school, many do offer some extended year programs for students that need it.

From a more practical perspective it is because teachers contracts specify they get that time off, and because the summer is used for the majority of school renovations, upgrades, and repair.

The last one is a big thing, majority of non emergency projects cannot be done when students are in the building bothe because it distracts students and becuase it may be dangerous. For instance taking the school's power offline to replace a transformer or water offline to replace pipes.

Every summer schools absolutely cram everything they can into those months to get the building ready to go for the next year.