First off, let me say that... people are weird.
And some of the more weird manifistations come out in sexual activity. Maybe because we're taught that sex is a taboo subject so, if you are already in the taboo region why not make it the place where any/all taboos can be broken?
Anyway, some of the people are looking for attention, some prefer to abdicate responsibility and some prefer to take on responsibility. Generally though, that only happens in limited circumstances. I think it very rare that you'd find someone that is "completely dominated" that would have anything to do with the BDSM lifesytle (Methinks you are in the realm of general domestic abuse there...). The "submissive"/slave may only be submissive while at home and in the bedroom. They may very well be "in charge" in their daily life. The Dominant/Master may very well be someone that is in subordiante positions during their daily life. It's roleplay and escapism.
There are a ton of studies on this subjecta nd a lot of it centers on "who is in control?".