The US Federal Government is prohibited from regulating interstate commerce (Constitution of the US). Thus the Federal government may not impose a local tax system on the individual states/local governments. Each local jurisdiction would have to enforce its local regulations and not all states would want to do so in concert. Thus unless a company is doing business in a particular state, they do not charge the "state" tax of the purchaser. The purchaser is technically supposed to pay the tax -- but obviously few do.
All this because of the original Federal vs. State argument when the country was formed in 1776!
Copyright Laws are the only "commerce" statutes that the Federal Government is allowed to enact (again in the original Constitution). It is interesting to note, that Copyright Laws were not originally enacted to protect the authors. Rather they were enacted to provide for the "Public Good." That is, if some type of copyright/patent protection is not given, then innovation would be stifled - leaving the public at a disadvantage. Today, Congress seems to be leaning more toward the author/inventor than to what is best for society at large. And of course technology has exacerbated the tension between the author and the public.