The usual way -- ostracization. If your main socialization is via the cult, then leaving means losing your friends. The stupidity of the cult means you have pushed away most other people.
I wasn't in a "cult" because I was once, a long time ago, in an Ultima fan community, but then was "ostracized" because I left. I just left. There were no consequences beyond the fact that I left. The problem with cults is that they don't "ostracize" you by virtue of failing to chase you down if you leave, they are cults because they do chase you down if you leave. Again, I have a hard time believing in "cults" when the "cult" in question literally doesn't even notice you. That's not a cult.
Or said less opaquely, most people have never truly experienced being ostracized in the way that a cult can. Most people have social ties outside of a group, and maintain those, even if their primary social ties are cut (as you’ve alluded to).
(I know we are arguing the same side of this point, I just wanted to emphasize it)