The benefit to reviewing is that it's frowned upon to publish in a conference/journal if you don't review for them. So academics are not just motivated by a "sense of duty": if they don't accept to review then it can become harder for them to publish (which has significant career benefits).

So if you don't review for closed-access venues (which I try to do), you are pressured into not publishing there: and not publishing in closed-access venues is difficult as most papers are collaborations, and usually your co-authors do not share your convictions.