Because the overwhelming majority of taxpayers do not read them or care to. Also, even though a taxpayer may not read papers, they hopefully still see the value in the progression of science, and allowing scientists to assume a certain level of background to the audience of their paper probably allows them to use their time on science.

Perhaps there's an opportunity for a motivated individual or group of individuals to make something to parse papers and help make it easier to read for those who don't have the proper background?