I should probably clarify and reword. I don't think mothers are explicitly forcing themselves out of maternal leave - I'm saying that it's not culturally acceptable to do so. The cultural implications are that you will likely miss out on job opportunities and thus wage increases.

The cultural components manifest themselves different ways:

As a biz owner: Why would you give a raise to someone who hasn't contributed anything to the company in the last 2-4 months?

As an employee: how would you (man or woman) feel if your equal peer didn't contribute anything for 2-4 months and got the same raise as you?