If that were true then how come existing social safety netting in the form of unemployment benefits haven't helped people stuck in this type of situation?

Unemployment benefits only kick in for people who have lost their job through no fault of their own, though state laws probably vary here. If I quit because my boss is a jerk, or I’m not being paid enough, or many other reasons, I am not eligible for unemployment. My employer has a lot of power, since finding a new job can take a lot of time that I may not have given a full workday and other responsibilities.

The vast majority of people who receive unemployment had an unplanned job loss, which already puts them at a disadvantage both planning-wise and psychologically. If instead you KNOW that you will receive income when choosing to leave a job, you are more able to prepare for how it will affect your lifestyle.

You don't qualify for unemployment benefits if you refuse to take a job due to unfavorable terms.