I'm a tenured professor, so my own position is not in danger if I boycott the likes of Elsevier (salary bonuses and grant money still would be, but at least I could still get a reasonable paycheck every month and keep a good standard of living). But the overwhelming majority of my publications are coauthored with PhD students or young postdocs who are fighting for stable positions, and their career depends on high-impact publications. If I decided to boycott for-profit journals, I would be throwing them under the bus.

In mathematics, most publications are single-author, and I suspect this (plus less reliance on funding to buy equipment) is an important factor why they are more successful than other disciplines fighting parasitic publishers, as established tenured professors might as well fight the fight. But for most disciplines, the lock-in is really difficult to escape...

Because ironically, and despite of all the good words, this is not what is typically rewarded by their universities.

In math they did. In other fields, they depend on funding from government agencies who support Elsevier.