It's made abundantly clear that cyberpsychosis isn't a real thing, it's just a propaganda term for people who are suffering lashing out in the way that society expects and practically worships, and then doubling down on it because they're afraid of the consequences and they've been propagandized into believing that they're inevitably going to become a monster anyways.

It's not so much that cyberpsychosis isn't real, right? Because all the people who get it (in the show) also happen to be serial killers for hire. They shouldn't be in the best of mental states period. But your instincts are right, only the angle isn't made clear in the show itself. In short, cyberpsychosis is a real mental condition and dissociative disorder. It comes about due to how humans have to cope with their bodies being substituted for artificial hyper chemicals and metal circuits. This everyone knows. What is less well known is that what makes people more or less capable of taking on chrome is to what degree their social relations are genuine, human, and existing at all. The reason why David is 'special' is mostly due to his relationship with his mom, which is almost an aberration in this world. That bit of humanity he lived through made him rare in Night City, and gave him a buffer against cyberpsychosis. That buffer then changed to Lucy.

Cyberpsychosis is in some ways a story about how a society has become so hypercapitalized, so hypercommodified, that it is no longer fit for humans. Being a cyborg is baseline because robots are the ideal. It's a story about alienation.

Nothing of humanity or human culture is left to be consumed by capitalism in a world where even your home appliances are a rentlord's market. That makes people at large ill equipped to understand cyberpsychosis. Humanity, community and all only exist in the cracks of society, it is the exception not the norm. It takes massive effort to keep it alive, and it is easily extinguished. So why would alienation, something that people probably don't even understand anymore, be the reason behind a well known mental condition? That is why the whole thing is coached in mysticism. 'Some metal is not meant to mix with meat', 'Chromejocks always reach their end', and so on. That's why David snaps back and forth depending on his proximity to Lucy. It's not fake. He really is delusional and in pain. That's also why Dorio's death hit Maine so hard, but he can still sorta keep it together when David shows up. What keeps people grounded are the genuine relations they've built around them. And those don't last long in this world, less so amongst merc operatives.