Over 70,000 synthetic chemicals are used commercially

This is a link to an interesting article underlying the many different diseases and disorders that may be caused by the use of synthetic chemicals. My question is, "If synthetic chemicals are created to mimic actual, natural chemicals then why are we creating fake ones? And is this the reason our bodies are reacting to these synthetic chemicals - because they are fakes?

http://womenshealth.about.com/cs/azhealthtopics/a/envtoxrephealth.htm