I had not seen this before, so excuse my tardiness.

Not blaming the victim here, but wouldn't the flight or fight mechanism kick in after a few hours or days? As for the hazers - something's going on if you have to beat your pledges for a month to make them love you. Psychologically speaking, what happens for either side to let this go on for so long??

No, not necessarily. The "I want to be part of the group" bit in the brain talks you into a lot, and so do your fellow pledges and even the actives. Not everyone is as susceptible to that sort of peer pressure, but more people are than you would think. And plenty of people who don't think they would stick around with any sort of abuse do. Hence cults, relationships and hazing.

/short version

I know people who have been in abusive relationships, and people who have been hazed mentally, but not someone who's been beaten physically over and over in a group setting. I guess because there is often physical evidence of the beatings/branding/kicking/etc., people might feel more likely to stand up or walk away.

I was thinking that it might be different, but thinking about the group dynamic, the desire to fit in is just as strong as the attachment in any one-on-one relationship can be.

Some people might be more likely to walk away, but I think most evidence of physical abuse is generally ignored or explained away in embarrassment. But when being "tough" and not letting down your brothers (in whatever sense) is presented as the ideal, people take more punishment. So in some ways, yeah the group dynamic might be more compelling, or just as compelling as the one on one. (I suspect it happens less frequently with women except when women are joining a predominantly male organization and then there's the aspect of 'keeping up' with the men. I don't have anything to back that up though.)