They're busy regulating life saving drugs. If we got rid of the FDA all together, there would be more competition for products like this, and companies would need to win the trust of consumers by providing real health study information.
Relying on companies to compete on the basis of maximizing customer health is... short-sighted at best.
Expecting the free market to do anything except optimize for profit is silly. And yes, that is profit at the expense of people's health.