Because there are other external pressures besides "Can people read my stuff?" Like it or not, for a junior scientist pursuing tenure, at this point in time, j

Because there are other external pressures besides "Can people read my stuff?" Like it or not, for a junior scientist pursuing tenure, at this point in time, journal prestige counts, and at the moment there's very few open journals with close to the prestige of Science/Nature/Cell/NEJM/Lancet/etc. in many fields. If I can get a paper in those, I'm doing it - because my science isn't going to be better served by not getting tenure because I took a principled stand. It's also offloading the costs. Libraries pay for journal subscriptions. Labs pay for open access publication fees. One is way easier to abstract away than the other, and even in large grants its hard to find the space to pay for multiple papers costing several thousand dollars each. I usually budget for one or two, but hopefully the research output from the project is higher than that.