This is a key question for anything, and I think the answer very often is "no". Cult or otherwise.
Sometimes it is the "no" itself that is the mistake, however, at least if you don't keep recursively asking the question.
Why do you actually trust that pharmaceuticals help disease better than colloidal silver? You personally might have investigated the matter to a sufficient degree of depth so that you could make a compelling argument, but lots of people don't have that kind of time, interest, or expertise.
Most people believe an awful lot of stuff because an awful lot of other people reassure them that it's the thing to believe and apply social pressure if they don't outwardly conform.
Sometimes there isn't any good answer except that you had to pick something in order to operate together, and this is what we ended up with. For instance, we build our lives around having seven days in a week, and all sorts of value gets imbued into things as a consequence, but if you go back and say: "hey, the Babylonians did this thing kinda because they only knew about the sun, moon, and planets out to Saturn, they thought seven was a cool number, and it kinda stuck, is that a good reason for it?" the answer is probably no. But we shouldn't avoid the tyranny of dividing things into weeks. Dividing into something is a good organizational principle, and as good communal creatures it's good for us to embrace and value whichever (arbitrary) organizational strategy we've got.
But in presenting an alternative, isn't it exactly this question that cults are asking prospective members? "Why do you believe that (mainstream) stuff? Is it actually for good reason?" If it is for good reason, the contrary claims from the cult have much less appeal than if it was all social anyway. The cult--like the rest of society--asks you to accept values without being fully aware of them.
The really tricky part is how to avoid pathological cases while making only surmountable demands of members of society. If we'd figured that out better, we wouldn't have to worry about cults so much.
When I was younger, I thought that simply embracing the scientific method and associated practices of thought and openness would be enough. But after seeing both the right ("facts don't care about your feelings" as a mantra stated confidently while...not...caring...about...facts) and the left ("In this house we believe...science is real" / "we're following the science (but if you ask me for citations I will insult you instead)") especially in the U.S. adopt the mantle of a rational evidence-based approach as a way to avoid taking a more rational evidence-based approach, I'm not so sure.
I do think it has to form the core of the approach, but I suspect that there needs to be some additional attention to human psychology that goes above and beyond what is needed when setting up a system that is workable for people who are already deeply committed to the process of figuring things out.