Not enough money in it. It's not something that is taken regularly for long term like ADHD meds, or something that pretty much everyone uses at some point like antibiotics. If psilocybin came in a pill, it'd be targeted to a relatively small group of people for occasional use.
it's also very easy to produce (grow) at home, further decreasing $$$ potential for big pharma.
> Not enough money in it
Source?
The simple answer seems sufficiently explanatory. Psilocybin is burended by its history as a recreational drug. That creates pockets of motivated advocates against it in a way that doesn't materialise for most drug candidates.