Science in the U.S. is taught backwards.
You generally start with biology, perhaps the most complex of all the sciences and the one that depends on every other science if it's to be understood.
You then proceed to chemistry, which is little more than memorization and explosions without a good knowledge of physics.
If you keep taking science classes, you may get to take some physics, which is the basis for all other physical sciences — certainly, biology and chemistry make little sense without physics.
Why is it like this? I don't really know, but I gather that the arrangement was codified in the U.S. immediately after World War Two, when physics enjoyed an unchallenged status among the sciences. Physics in the first half of the century had triumphed in the terms that science itself values most — in its predictive capacity and its ability to sort out basic questions about existence — but American culture also saw physics as triumphant militarily and politically, and as the basis for atomic power and atomic weapons.
As a result, the attitude was that little tykes were not yet fit for the revelation of such a Great Secret. And teaching little children had (and still has) a low social status. Teachers trained in physics could just as well do other high-status jobs, unlike those with training in biology who would otherwise be doing various "helping professions" (women's work, you might say). So physicists taught the young adults, and biologists dealt with the children.
Again, this is the story I've gathered. In any case, an "historical" or cultural explanation of this sort has got to be the right explanation. No more rational, functional explanation is likely, given that the current arrangement makes so little sense and its results are so damaging.
