Part of the problem with herbal medicines in the U.S. is that the FDA has a separate category for them. Congress passed the "Dietary Supplement" bill in 1994, which classifies herbs as dietary supplements, not drugs. As such, they cannot be regulated by the FDA (or any other government agency), as long as they are marketed without specific health claims.
So why not make the claims anyway? Why hasn't e.g. Glaxo or Pfizer done some research and marketed (insert favorite herbal supplement here) as a drug with real health claims? The answer is in the economics of the process: it is profoundly expensive and labor intensive to bring a drug to market under FDA rules. There is the R&D cost, plus three phases of clinical trials (these literally take years and the execution cost is phenomenally high), then you have to file an NDA ("new drug application") and wait for approval, which the FDA may or may not give. Depending on the drug and who you believe, estimates range from around $50-$100 million to well north of $250 million in some cases for the cost of the entire process.
This is, in a sense, what patent protection is for: it allows companies to recoup some of the enormous costs they have racked up by being given a temporary monopoly to sell the drug. (It is also one reason why prescription drugs, which cost nothing to produce on the margin, cost so much.) No "big pharma" company is going to pony up all that money if the drug can be sold as an herbal supplement by other companies that didn't incur those costs.
The irony here is that while most herbal supplements are completely ineffective (or even harmful), there are almost certainly several that have genuine pharmaceutical effects that could be isolated, refined, and brought into real medical use. We will never know which ones, though, until the economic incentives are changed, or government subsidizes the drug discovery process.