I work with people with various levels of mental disabilities and developmental disorders. Given proper instruction, they can be hard and motivated workers with a surprising level of skill within a sufficiently narrow field. However, many of them lack initiative (or if they have initiative, they lack the comprehension to utilize it safely) and are often unable to live independently without someone taking care of them.

So yeah, in a fictional setting that is harsh, brutal, pragmatic and barbaric, becoming a thrall is a likely fate for such people. Heck, it can be close enough to that in the real world as well.

Of course, this does not explain why trained, professional soldiers in the game are still complete morons with no sense of self-preservation.