Mainly because there are a variety of different chemicals/cannabinoid at play when cannabis is ingested traditionally, not just one "purest" form of THC in high doses which is what you find in THC pills.
Not to say it can't be replicated/refined into pill form correctly, but my guess is it just won't be.
I don't think there will be a market for high quality pot-pills large enough to justify the massive investment needed to do it right when a better alternative (natural cannabis) already exists on the market and can be ingested in ways people generally enjoy (vapor, snacks, teas, etc).
That is, companies probably aren't going to increase revenues by increasing the quality of a product that already exists because the people who already use THC pills can't be "re-converted", and enough of the people who don't use THC pills, but are in the market for medicinal THC, probably won't make the switch.
I can't find the link now, but I recently read a paper from Israel about how the effects of CBD acted in a bell curve when administered alone, but were much more linear when taken as part of a whole plant.
There are many cannabinoids, and they do often seem to work best when taken together.
Isolating salicylic acid from willow bark and then a century later synthesizing acetylsalicylic from salicylic acid were all major achievements in medicine. It wasn't some sort of evil scheme.
I'm also guessing synthesizing acetylsalicylic in a lab is better than growing willow trees in almost every way. Less resource intensive, more environmentally friendly, cheaper, easier to scale, and faster production.
The "natural is always better" (for varying definitions or "natural", of course!) argument is silly.
"heavy refinement, solvents, chemical processes" are all just scare words. They don't mean anything useful. I can use those words to accurately describe cooking my dinner.
The process of taking willow bark and turning it into a consumable product is not minimal, whereas taking cannabis and turning it into a consumable product is very minimal.
From a practicality standpoint, it is simply logical that medicine derived from cannabis is superior to willow bark.
How did you get this idea? Scrape some bark off a tree and steep it in hot water. You now have a consumable product.