Because schools are overburdened. As a [financially-conscious] member of society, I am frustrated with the lack of financial preparation our students get. As a teacher, I don't know where we'd get the money and teachers for it. Schools are strapped as it is, and federal mandates like NCLB and corresponding state mandates have made it so that the focus of the schools, in the past ten years more than ever, has turned to be very heavily oriented towards math and English, and, to some degree, science. If scores aren't high enough in these areas, schools lose what little funding they have and receive a variety of other sanctions. Because of that, funding for programs like art, music, technology, and financial education have been significantly reduced--if we're not tested over it, apparently it doesn't matter.
Also feeding into this is our constant need to compare ourselves to other nations in those areas. However, those who do the comparing fail to take into account that we are the only nation who educations (and thus tests) every student. And if we are going to educate every student, we need to, like you indicate, educate them for the world with things like a financial education.