"So they took a bunch of people prone to mystical experiences and found when you give them hallucinogenics that they have further mystical experiences?"
The people they recruit aren't really much more prone to mystical experiences than anyone else, they're just trying to find volunteers who are psychologically stable and have a support network in case things go poorly.
There's only 20 or so people in the field, and they all help each other to get their studies approved. I think that Rick Strassman is the one who came up with this idea, and he writes about it a bit in his book DMT: The Spirit Molecule. Then all the other investigators started doing this as well. Roland Griffiths may have also talked about this in one of his talks, I forget.
The people they recruit aren't really much more prone to mystical experiences than anyone else, they're just trying to find volunteers who are psychologically stable and have a support network in case things go poorly.