Wait what?
Does this include being in a small 'confined' space like a car or 'contained' in their treatment facility or only like active imprisonment and keeping them someplace they have no desire to be?
For the first, Changelings are inherently sterile. Their changes are not consistently designed, and they are simply too different internally to be compatible with humans or each other. As noted, most of those eho escape were the ones who were tortured, and the lingering trauma almost inevitably makes them... let's call it psychologically different, because most of the ones stable enough to get out are not what is implied by the term 'insane'. Some of them
are insane, however, which can lead to them deciding it's fine to lure a relatively careless 'Fae' that they know they can steal from into capturing people, so they can 'rescue' them and gain more Changelings. Those particular ones don't tend to last long, because individuals insane enough to do that have trouble hiding what they're doing, and their 'children' generally don't appreciate it.
...Or, as others have pointed out, there's also the fact that one of the options available to Changelings who have raised the brute force of their power as high as they can is to convert themselves into a Fae.
As for the confinement? Every Changeling was stuck in a role, acting to a character over and over in the same spot, frequently for years, forced back whenever they tried to break character or leave. There is often variation in the situation, but almost universally no 'character development'. If they genuinely believe they're free to go, they don't mind staying in one place for a while, to the point that there have been some of them who calmly accepted a holding cell for a day when completely confident they would be pardoned at trial.
...Of course, the main example of that behavior snapped and killed twenty people, including himself, when he was then judged guilty by the Masses court in question, but, the point stands.
