I'm a european pharmacists with some interest in this, just going to add my 2 eurocents.
I'm stepping a bit further into speculation here but it appears that there is a social aspect here on not wanting to be seen as a quack for prescribing "weed in a pill" when there are a number of other schedule 3 drugs which are widely accepted to treat pain, even though those pain pills contain opiates. Cannabinoids have been shown to treat issues besides pain but again there are other drugs on the same level of controls or lower that are more socially acceptable..
I think there is a certain level of stigma associated with cannabinoids, same as any other abusable drug but my theory is that putting it in a pill would make it more socially acceptable. Opiates and central stimulants in powder form will immediately mark you as a junkie, but in a pill then it's medication. Completely different things. I believe the main difference is the experience of prescribers. Opiates and other drugs of abuse have been used for therapeutic purposes for a very long time and the way they work is well understood. Research on cannabis for therapeutic purposes is fairly recent, as most resources have been spent on trying to prove its harm. So I suppose we know a bit about the harmful effects of cannabinoids but the beneficial effects are not that documented. So as a prescriber you would have the option of going with something well known all-around, or take a shot in the dark. This will likely change, but it's going to take some time.
Humans are unlike many other species of animals out there. We've got such a varied genome that drugs can have a wide variety of effects. Animals like horses, cattle, and domestic cats don't have such variation so when drugs are tested on animals they don't always tell the whole story. The only animal that seems to be as widely varied as humans are dogs..
Not really true, animals are in general as diverse as humans. Exceptions would be domesticated animals where individuals within a certain breed are highly similar in their genetic make-up due to human intervention (breeding). It's really a matter of statistics. Drug trials on animals are performed on relatively few individuals of similar genetic make-up so that effects are predicable. Animals studies are used to determine safety and as a rough measure of dosage. But this is just to get a hint on what substances and doses can be used in humans. You still need large scale studies on humans (thousands or more) to determine if a drug is efficacious compared to placebo and to determine the side-effects. Even then you cannot predict all possible side-effects because some are very rare. All in all, a certain recommended dosage is valid on a population level, but not always on an individual level.
If put in a pill form the marijuana would have to be in a wide variety of dosages and/or the physicians may have to prescribe a rather unrealistic number of pills for some people. This would make the regulation difficult and make them expensive. Marijuana in its natural form is easy to meter in that it is dilute, just take a bite of a marijuana cookie if that is all you need or eat the whole thing. Marijuana is naturally cheap to produce, it's a plant that grows like a weed, processing it to a pill form would make it expensive..
This is also true for opiates. Morphine occurs naturally. However, marijuana in its natural form is decidely not easy to meter or dilute because you have no idea of how strong the plant will be. Smoking or baking cannabis plant extracts is extremely difficult to dose. A pill would be standardised. You would always know which amounts of active substances you have in it (THC/CBD ratio has been shown to be very important for therapeutic effects). You can standardise this in a plant, but you won't know what the patient will do with it and how that will affect the dosage. I agree that selling cannabinoids in pill form would be more expensive than plant parts, but not that much. The therapeutic benefits of actually knowing the dose would be well worth it.
Probably the biggest reason that drug companies don't just put it in pill form is that there would be no profit in it. It would take only a minute for someone to see that the pills they are prescribed are just the same thing the stoner on the street corner is selling. Drug companies cannot compete with that. Then this quickly turns to politics. For a drug company to sell a drug derived from marijuana on the market they'd have to lobby the DEA to reschedule the drug. Since there is now a large gray market for this in many states the big drug companies know they cannot act quickly enough to get any profit from it. Their potential customers would be quickly grabbed up by the existing marijuana dealers. Those taking what the big companies are offering now might just switch. They stand to only lose from making "weed in a pill". Therefore they fight to keep the status quo.
I don't think competing with black market dealers is the main problem here, that can be fairly effectively dealt with through law. The problem with profitability probably stems from the need of very costly studies to prove the beneficial effects beyond doubt. As these substances cannot be patented, no monopoly can be granted and prices cannot be raised without risk of being undercut by a competitor (i.e. another drug company). So actual standardized medicinal marijuana (i.e. cannabinoids in tablets/capsules) is not very attractive to the pharmaceutical industry because it's cheaper to fight it than to try and sell it. This probably won't change until the regular drug sales starts to really plummet, and I have a hard time seeing that as long as medical marijuana is comprised of plant parts.