Because there is a self-aware, behavioral component to not just dissociative events, but to most maladaptive psychiatric disorders. This component may be small or large - the fact that we've medicalized the disorder simply says that the overall pattern of behavior is causing severe problems, nothing about cause or self-awareness. The people afflicted with them are not simply normal people who are possessed by a random event, like a lightning strike; Their expectations, moods, and desires play a role in the onset of crises. The temptation is to split their agency, their consciousness and their participation into a boolean and say they either have it or they don't have it, that psychiatric disorders must be entirely out of someone's control before we give them a pass on "But it's their fault" and "Just snap out of it!". But allocating problems this severe to the realm of personal responsibility and purely voluntary deviancy is not very helpful to anyone.

The most infuriating thing about having a family member with a personality disorder, my own dive into ab-psych, is that they can turn it on and off in certain contexts, they can be triggered and they can avoid being triggered, but not in a context where you need them to, where you beg them to. It is a way in which they are acting, an unhealthy coping mechanism and pattern of thinking that colors all their thoughts - but it is not something that they can't control or observe at all, only something that they don't usually control or observe well enough to avoid causing problems. There is no clear line between their disorder and their voluntary behavior and their personality, it is one unified whole.

Do we hate them for that? What's the use? Who does that help?

Before we started medicalizing these things, we relied on public shaming and ostracism to control them until the person's behavior got so out of control they became a danger. While it's a cruel way of doing things that we have wholeheartedly rejected, it was surely at least somewhat effective, because of this voluntary component, because telling yourself "Keep it together" is not a 0% effective treatment technique. The country did not suffer from a 26% mental illness rate in times past, and I think it's foolish to assume this represents pure underreporting. Mental illness at least partially conforms to the outlines of the society which defines it - immigrants to the US find themselves substantially healthier than the US on arrival, but develop statistically average rates of US mental illness over time; Symptoms shift from whatever is recognized in their home country to symptoms more in line with US-recognized disorders.

We have decided, rightly or wrongly, that classifying these things according to an insurance-industry handbook, and doling out pills like candy, and pretending these people have no control whatsoever over their disorder in order to relieve them of social responsibility for it, is the correct approach. While the truth is more complex, I tend to find this more humane than some other national attitudes towards the topic, and I have no better approach to offer.

Edit: The problem is, when you acknowledge that these things are maladaptive and problematic behaviors that someone presently does not control well, rather than some kind of seizure that completely robs one of consciousness... you get people popping up who want to go back to the boolean definition again and deny there's any problem whatsoever, claim that they're doing this for attention and faking it and horrible hoaxers who are sucking the precious fluids from our society... There may be too many douchebags out there for a nuanced view to prevail. I see this post already has one, https://news.ycombinator.com/item?id=9147386 . The problem is the boolean: A 100% controlled premeditated affectation must represent a malicious manipulator, the theory goes, while a 0% controlled mental illness strikes a victim, all victims, like a bus hitting an innocent pedestrian. But the mind isn't so simple, and the onus of personal responsibility is not a cure-all.