Because many schools are already crowded. Where would one put the kids? There are guidelines for how many square feet per child in our state regs. There's also an insurance wrinkle. Many parents who would prefer high-end childcare wouldn't want the preschoolers in the same building and sharing the bathrooms with older kids, so you have an additional logistical cost with creating/retrofitting plumbing for children of that height/age. You would also have families who want closer-controlled security, who wouldn't want the same entry as hundreds of elementary school children.
Need I mention naptime and the requirements for quiet? Most CDCs (child development centers) have a set hours for the napping throughout the building, where all the groups are either sleeping or offering quiet play options for non-sleepers. Kids outside at recess would be *noisy*. And then we also have the issue of play equipment outside which is too larger for their small bodies and therefore,unsafe. So, then, other options/areas must be added to be age-appropriate and offer quality play opportunities.
Plus, elementary schools are run as a goverment entity, taxpayer funds, no charge. Having a for-pay program means that the school is taking in money. Or, if they lease some rooms to a company who runs this sort of preschool and has the money to pay that, there are, again, other wrinkles.
The short answer, really, is that this would have to be a top-down change from the government, either state or federal. I personally think that it would be better if we had higher taxes as well as the benefits which come with it (like single-payer health care, believe it or not, if you talk to Canadians, they don't complain) and free preschool for all. There are structures for this in other countries, but until the US citizens feel comfortable paying more taxes for more benefits and would pressure the gov't to spend less on outside ventures (military), this isn't going to happen. Here in Portland, some of our 'old' schools have been repurposed for Head Start; that said, we have very high numbers in our classrooms and our local public schools are a mess in regard to organization and leadership from the superintendent down. As someone who worked in early childhood education for 20 years, I'd love to see socialized childcare/preschools; that said, the best places for youngsters to learn, in my opinion, is still home-based preschools. Smaller groups, a more homey atmosphere, and less focus on 'school' (academic), more focus on play being the work of the child. I would never have sent my son to a 'school' atmosphere preschool, personally. His preschool had chickens and ducks, a garden, small treehouse, a corner of the garden for digging, etc-- sort of like a small farm, cozy fires and storytimes....so much richness.