Also, the process of manufacturing powdered extracts is very expensive. The amount of active compound placed into a capsule may not contain the same amount of active ingredients as a liquid preparation.
Lastly, liquid preparation methods have been well tested over centuries. In all societies, medicinal herbs were usually prepared as a liquid preparation. Did you know that the history of the effectiveness of the herbs is based on liquid herbal preparations?
The quantity and quality of active compounds in a plant source varies according to many factors: e.g. where it is grown, the soil conditions, the weather conditions, etc. To illustrate this point, it is well known that in the production of wine, the flavor of the grape changes from year to year and is also dependent on where the grape is grown (e.g. A Merlot grape grown in CA will produce a wine which has a different flavor from one that is grown in Ontario). The same issues apply to medicinal herbs. Since the quantity of active compounds found in a single herb changes from year to year it is quite conceivable that the efficacy will have great variation.
In an exact formulation (combination), the chances of all of the extracts in the combination having lower amounts of the active ingredient is relatively small; so from batch to batch we do not expect great changes in the efficacy of the product. Also, formulations include herbs that negate any potential side effects of other herbs. This cannot occur in single herb products. In addition, some herbs in the formula are present to enhance the absorption of the active compounds thereby producing a product with superior efficacy and cost efficiency (use 1 formulation vs. 3 or more individual products).