Yeah, I hear that "4x as much" figure a lot but few people seem to actually charge that. Instead, what actually happens for most people is that (for example) they are covered under their spouse's healthcare policy, so they don't need to cover the cost of benefits.
Historically, a full-time job with benefits was designed to be held by a married man who was worth that kind of compensation because he gave his all at work, went home, collapsed into a chair and said "Woman, get me a beer!" and "What's for dinner?" Then he slept like the dead.
Research shows women with children typically cannot give their all to a job like that. They get stuck in Pink Collar jobs that preserve time and energy for the "second shift" of cooking dinner, doing housework, etc. They need to be able to get up in the middle of the night if their kid is sick and so forth.
People are marrying later, having fewer kids and so on. I routinely see comments online about the downsides to that, for example how it's impossible to work full-time and cook from scratch to feed yourself properly as a single person. Cooking for one is too time consuming and labor intensive. Home cooking makes the most sense when one person is cooking for an entire family. This leaves a lot of people living on takeout and the like, which amounts to overpriced junk food in most cases.
Employers need to get enough value out of the transaction to make their business viable. They can't pay a living wage with benefits out of the goodness of their hearts. And we no longer de facto expect the wife's labor in taking care of her man to be a hidden part of the deal for ensuring that the employer gets enough value out of the deal.
I think these social changes are major driving factors in the gig economy trend. It puts the onus on the worker to figure out how to accomplish enough to pay their bills at a price that makes sense to the employer.
And part of the answer is that supplementing your income with low paid, small tasks using scraps of time that you previously could not have used to try to make money helps that equation make sense for both parties.
