All good in theory, but insulin is a protein, so it is made directly by translation of RNA. THC is the product of a (probably complex) biochemical pathway involving a number of enzymes which start with something (relatively) simple and convert it steps into THC. In other words, the plants are like little drug labs, carrying out the synthesis in steps, taking the product of one enzymatic reaction to use in the next. Difference is, they dont have too many troubles with precursor aquisition... lucky bloody plants...
Anyway, to have bacteria produce THC, you'd have to isolate all the genes from the plant that encode the neccessary enzymes, and then manage to get them all onto plasmids, and then get all of the plasmids made into the same bacterium, and then get them to express the genes, and then isolate the product from a mixture of everything else the cells produce... it'd be ugly. And expensive - enzymes and other biochemical reagents needed for this sort of thing can cost thousands of dollars per gram.
It's relatively easy to get bacteria to produce proteins (eg. insulin) for you. Getting simpler molecues is where things are much harder.
