For the vast majority of herbal preparations, it is because there is no need to test the preparation (there are already modern preparations that serve the same purpose that are highly effective), or because it would be cost prohibitive to test the preparation out of pure curiosity. (Basic science is something that industry hates, because it does not guarantee a return on investment, AND is expensive. Assigning the researchers to find a profitable product is MUCH more amenable to getting funding.)
That said, more often than not, herbal preparations are found to be effective, but not MORE effective than the commercial preparations already available. Some have novel methods of action that spur additional research and open new avenues of treatment with modern preparations, but that's a rarity.
The novelty of some herbal preparations comes into play when you have things like MRSA, which comes into being because people abuse the shit out of modern antibiotics, producing supergerms.
There are some herbal substances that have been shown to be effective against MRSA, and those substances are quite easily obtained. (from the study, lemon grass oil was so effective at inhibiting MRSA, that it inhibited all activity on the test plate. Lemon grass is a common food flavoring used in southeast asian cuisine, and can be purchased at asian stores for a pittance.) For a topical MRSA infection (MRSA on the skin) topical application of one of the tested essential oils could prove very useful for treatment, without the GI complications of using IV vancomycin. (the heavy hitting commercial antibiotic used for aggressive MRSA infections.)
Note that the reason for the study is given right there in the preamble of the study-- Only 10 new antibiotics have entered the scene since the adaptation of golden staph into MRSA, and MRSA is a sufficiently serious infection that new strategies for treating it are very much needed immediately. There were reports of activity with volatile plant oil isolates in regard to inhibition of growth, which necessitated the study.
Note-- many old (dark age) preparations for stopping infections make use of volatile oils, many of which are included in the table, and which showed efficacy.
The important thing here is that many herbal preparations *ARE* effective. The problem is that herbal preparations are not reliably potent, not easily dosed, and rife with counterfeit products.
There's also the unfortunate popular misconception that herbal preparations belong in the same category as hoodoo bullshit like homeopathy; that herbal preparations are basically just placebos. That's simply not true in the majority of cases.