Can't speak for anyone besides myself, obviously, but for me - it's because MP tends to become the focus, rather than another way of enjoying the game. It is also more open to abuse by things such as microtransactions.
Look at Dragon Age: Inquisition...the developers claim that MP actually improved the SP experience, although the combat in Inquisition is - in my view - the worst of the three games in that series.
Perhaps if there was story mode co-op, people here might be more receptive (I don't know that they would, but maybe). But it seems that - other than Sword Coast Legends - every game that features MP does so as a separate mode.
Would Obsidian consider doing campaign co-op as opposed to a separate MP mode? Don't know, but - and while PoE is clearly running against the current of industry trends - it would seem unlikely.
For me, if it includes a 'separate' MP mode, whether 'not required' - like Mass Effect 3, or 'not required, but totally preferred' like Dragon Age: Inquisition, I would be against it, merely on principle. If it includes a campaign co-op mode, which allows another way of enjoying the main story....sure, why not?