Gender roles and families allow the existence of both emotional states within one entity - it is possible for a family to be made up of both a very human side and a very aggressive side. This maximises the success potential of a family unit - one half can be ambitiously working towards acquiring more, while the other can be ambitiously working towards building closer family ties and emotional support.
When both parts of the family are aggressively working towards acquiring more, the softness of a family gets lost. There is less emotional dependence, less of one person doing one thing, and more of two people who have a lot of potential to be at loggerheads with one another.
Raising a family and acquiring a lot of money require completely different emotional outlooks, and I believe that there are not many people who can make the switch from roughriding for money to being tender at home.
Families need two different people fulfilling different roles to be successful. That's the evolutionary method, that's the method that is culturally agnostic and that has taken us as humans this far. Intellectually trying to force a different system to exist is causing unhappy societies and break-ups of family units that used to be happy.
When couples say the other half completes them, they mean that the other person fills in some parts that they lack, they don't mean that the other is exactly the same as they are.