Because some crazy people want enforced absolute gender equality in all things, and screw what the people doing the work actually want.
It is well known that, biologically, men are, on average, more interested in high-risk/high-reward careers. That doesn't mean that all males are, or that no females are, but on average, that sort of career is going to have a lot more guys interested, just by the nature of it, and that is not primarily a social difference. That category includes "risk of physical danger" jobs like firefighting and police work, yes, but it also includes "risk of no free time and stress burnout" jobs like high finance and certain types of programming work, where you're making mad bank, but good luck finding time to enjoy it.
Thus, it would make perfect sense to me that if you separate out hours worked, men and women would be making roughly equal pay, but if you didn't, men would make more. Not because they're being paid per hour, nor because they're being "offered" longer hours, but mostly because they're working more jobs where longer hours are just *expected*.
I'm male, and working at a software company. I've been told I could probably make a lot more with the same experience if I worked at a different company, but I *like* working at a company where flexible hours are the norm, and working a standard 40 hours a week is expected. I know people who make a lot more, but working upwards of 50-100% more hours a week. No thanks. Girls (again, on average) have the right of it.