But most companies want to pay as little as possible, and worker want paid as much
Men are more willing to negotiate for a higher wage. Not all of them, but enough that usually explains this effect for entry level jobs. And a very sizable amount take at least a few years out of work for kids, and that gap usually causes them to earn less when they go back. And that set back keeps them out of the highest positions most of the time, which is why this gap is higher as age increases.