@remiska
It's an idol mentality. As a budding actress, they, the management, would want her to be able to grab as many eyes and minds as possible. And, sadly, with how they've gone about it up till now is, "single, virginal, angelic, and untainted".
This stigma, as well as the inability to really see their significant other because of obligations- work, meet-ups, interviews, etc- makes relationships really touch-and-go. Then, with some more dubious management, they will pair the girl with a guy to make them out to be a "celebrity couple". So then you have that looming as well.
This is information from a good few years back. It may be vastly different now.