Isn't a portion of the issues that human psykers have that they don't really know the dangers of the Warp when they start poking around and thus get suckered by daemons?
That plus human psykers usually have no form of training to protect themselves against the stuff the warp throws at them so it's like they have a constant sanity check, and few people are willing to help those psykers learn how to shield their minds for fear that they will "grow to be a threat".
(Though I suppose the ability to live life without trying to scream and claw your eyes out all the time makes you
something of a threat in that you can actually think straight.)
Then of course there is the standard practice of finding psykers and putting them in isolation cells for years on end without any form of human contact while awaiting the blackships, which then transfer them to an isolation cell again. Except this time they can feel the dispair and insanity of everyone who is on the ship along with the echoes of past inhabitants
while traveling in the warp.
Also the constant beatings and torture as a means of "beating the sorcery" out of an individual or just because "he/she is a witch, so it's okay."
Plus psykers are trained to believe that they are worthless worms, unfit to be called human. That they must give up all self will and follow orders.
Because that's what you want with your guys who are connected to the warp, a weak will that follows orders from anything stronger then itself and a belief that it's only purpose is submerging itself with the greater whole.
Isolation chambers, beatings, torture, social isolation, not being trained in methods to defend your sanity, being trained to believe you're a worthless piece of trash, and never being told anything about the warp because it might "strengthen" the witch. It all leads to really messed up people who have really weak minds and eager to please those they believes are better then themselves. Deamon fodder basically.
I mean beatings and torture
don't produce healthy minds able to defend themselves against possession. Who knew?