Because career advancement requires publication in peer-reviewed, quality journals in a relevant field. The people at the top aren't generally keen to delegitimize the reason they are at the top. :) It's hard to fight that battle on top of all the other maneuvering and petty bullshit an academic already has to put up with.

This is why open-access tend to be sudden revolts of entire subfields. Everyone gets together and says, fuck it, we know how to judge ourselves, so let's stop signing over copyright.