1) We recognize that Summer break was never meant to be time off (it's time when you needed all hands in the field and wouldn't have sent your kids to school anyway), and that do-nothing, responsibility-free childhoods are a rather recent human development. However, it's still healthy for kids to have to learn through play and be free to pursue things on their own. They need the break.
2) We recognize that the break puts a burden on parents to find activities, day care, or camps during the summer. However, it also provides a huge block of time for lengthy family vacations, which would otherwise be impossible to schedule, or even costlier because all kids get the same three week-long mini-breaks. This is good for the entire family's health and quality of life.
3) We recognize that other countries are lapping us in education. But we also have to recognize that that has nothing to do with time or money spent per student. We invest more per student than pretty much any other country, but we get worse results. That's because the fundamental changes to in teaching methods that we've made over the past 50 years have been for the worse, and other countries have made changes for the better.
4) No one wants to pay teachers for the nine months of work that they do already. More time means more cost, which no district's taxpayers are going to pay.
Ending Summer break is another costly distraction from the real problems: many teachers are unqualified for lack of training or materials, all teacher now teach mechanically to standardized tests which distract from the actual material, and many students are never going to achieve their full potential unless we first address some very hard, very real social problems first.