The worker could, of course, quit. But this does not make the relationship equal when it comes to power. If she is receiving an economic rent she would penalize herself by quitting, and her employer would just replace her with someone currently unemployed.
This power dynamic is extremely important for monopsonies where factor mobility is low, as workers cannot easily relocate due to transportation costs, familial reasons, the location of their assets, etc. Thus, immobile factors are extremely dependent on their employer. In addition, as we learned in class, immobile factors feel the effects of changes in trade the most (compared to employers, mobile factors), either for worse or for better.