Because swallowing a horse pill when you are suffering from nausea isn't as much fun as it's cracked up to be, and because patients have fine control over the how much unprocessed marijuana they ingest, compared to being codwhalloped by enough cannanoid to choke a horse. As to "facsimile", why not dig up some testimony from someone this "medicine" has been fobbed off on? This drug was frogmarched thru the FDA tests at the behest of the DEA--it would never have been allowed on the market otherwise, it isn't spectacularly effective, and it is unduly dangerous compared to what it can accompish. It exists because the DEA refused to follow its own rules and reschedule marijuana as its own Judge-Advocate Young instructed it following a year of intensive judicial review of the scientifically verifiable effects of marijuana.
Are you not fascinated by the logic behind unprocessed marijuana (whose physiological effects have never been known to kill anyone) being a schedule 1 drug, of no possible redeeming medical value, whereas marinol (which actually does kill patients) can be had by prescription?