My guess: they (as in, US carriers) follow FEBO because they think it's a "fair" thing to do.
The Last shall be the First, etc...
A possible legacy from the olden days, maybe an assumption that Mr 1A and Mrs 1B are going to be 1A and 1B on their way back. Everything in order and everyone in their god determined place.
Except that, in the modern days, Mr and Mrs 1A and 1B who got last choice on their inbound flight weren't able to book the same seats on the way back because of crafty FTers who had figured it all out, and thus they also got last choice on the way back.
A sad story where obsolete chivalry makes things worse when trying to make them better, thanks to the internet. (Still only a blind guess, mind
)