Why do people use Facebook or Twitter or Instagram? Convenience mostly, and some nice features that emerge from centralisation. People, including academics, are about as willing to give up their copyright in order to be published as your average Joe is willing to give up their privacy by using Facebook.

What journals and publishers provide are: esteem, convenience, trust, correctness, and, and this might sound outrageous in the age of the bazaar, but exclusivity. There is an awful lot of crap out there. In my opinion, the best feature publishers (open access or otherwise) provide is being a filter for all the noise of the internet. Some journals are better at this than others and reputation is hard earnt and easily lost.

Also, I can't speak for all OA journals, but the Impact Factor is considered an awful metric and avoided. There is the rise of so-called 'alt-metrics', but I've yet to see anything less-awful come out of that yet.

Some people do, but most don't because they don't really care about science or spreading information, but just getting an easier way to progress in their career. And the system is gatekeeped mostly by such people, so in a lot of fields one is at considerable disadvantage if they care about things other than selfish career development.

What baffles me is why these people choose academia. It's a really uncertain and laborious way to even sustain oneself, let alone succeed.