A better question to ask is - when researchers can publish anywhere (including open source journals) they choose to publish in journals with paid access.
But if you ask why they choose to, they mostly don't have that much choice. Often, rules say "publish in magazine with impact factor at least X" and that's it. The publishers are the gatekeepers of that.
But if you ask why they choose to, they mostly don't have that much choice. Often, rules say "publish in magazine with impact factor at least X" and that's it. The publishers are the gatekeepers of that.