Isn't it because just about anyone with arms can do it whereas most people can't do what I do and most people can't do brain surgery etc.
This is an interesting one from Grazia. Basically, I think she has nailed it.
But there are people like me...
I could have stayed in work at £60K, paid a nanny £20K, and pocketed the £40K differences (pretending we live in a world with taxes, ha, ha ha! but trying to keep this simple.) I baulked, because to me (and this is a very personal opinion) I thought I could do it better than someone else. I felt that the someone else wouldn't have as much "skin in the game," as I did. I also wanted my children interacting all day with someone as intelligent and as educated as me, which I wasn't so sure I could find for less than the £60K I was earning.
Now, I am very happy content with the children I have, our relationship, and how they are turning out to be. So, I could take that as a sign that I made the "right choice." On the other hand, for all I know, it would have all turned out the same or even better if I had done something else. It's only now that the children are growing older, that I am coming out of a fog of intense emotion, that I can even consider things in the round.
In my case, I have a few job interviews coming up, fingers crossed! And perhaps this decade as a SAHM will just be one, lovely, much appreciated chapter in my life. If so, I wont' have any regrets.
But, having lived it and made the choice, I will admit you have to pay the price in your career and financial outlook. And I will admit that the children of full time WOHM seem to also be doing OK. In truth, I don't know many dual fulltime, workoutside the home couples. So many are able to do 4 days each or work at home a bit, or mum works 3 days. So maybe we are already seeing the change in work culture happen before our eyes.
(As I have said upthread, the career break and the chance for perspective and introspection has been invaluable to me as a person and my development. Clearly this is very much an indulgence not afforded to most people.)