Is your question really why am I doing so much more than my man? I just think the question is a bit more general than we can really consider. All women are certainly not doing more than all men.
I do think society is a factor and that women feel a certain strain and stress related to keeping a home, raising a family and working. I acknowledge men have a different set of stressors and strains in their life but I am no expert on those- I'm a woman!
I did have an interesting conversation with one of my mother's friends. She said that my generation is great-- we seem to set more boundaries and require our spouses to chip in. She discussed how hard it was for her to work, clean the house, raise the kids and work. She felt like even though she worked just as much as her husband, it was still expected she fulfill all of those tasks as if she wasn't working.
Society definitely influences us but it does not define who we are and our relationships. If we want a more balanced relationship-- we can just go and create one. If we accept that this is what people expect, this is how people are, and this is how we should be-- then we cannot expect anything to be different. Personally, I try to make sure that what I have in my life is working for me. I could not handle all that I am doing if it wasn't. I don't think I have time for anything working against me.
To answer your question more directly- why this is happening to some women (because I realize many women out there are in that situation) is because they are in a relationship and a situation that they need to actively change so it fits their needs and supports them more. I can't see how the man in that set up is going to recognize things not working unless the woman speaks up- that's quite a sweet deal! Relationships are best when everyone is happy so asking the man to help with certain things is better for everyone. Now, I realize there are a lot of cultural and personal influences unique to each relationship and that simply asking for help may not work. That means that particular person has more to work with because you should not give up when you feel as though you are falling apart, are unsupported and are stressed out. Hopefully the other person will realize that holding onto their rigid beliefs regarding roles in a marriage is more trouble than accepting their spouse needs their help.