My answer is because it's not a 'traditional' job. There's no going to an office with co-workers and coffee breaks and vacation time. Now, it IS a job in that I get paid for my writing (which certainly helps with expensive children and an old house) but I really think it's because it's a stay-at-home job that it gets short shrift.
True, you CAN write anywhere. If you've got a pen and paper then you're golden. But you need time. And for me, solitude. I have discovered that if I'm just working out plot or background then I can listen to the radio (my wonderful Classic FM from London, England), but if I'm REALLY writing then it must be quiet - mainly because I mutter as I write and I need to hear myself! This habit tends to garner odd looks if I'm in a public place. I didn't mind being looked at when I was acting on stage - and if I could get a good laugh then I was really cookin' but in real life I'm not that forward. I have trouble speaking to strangers.
Everything that I've read about writing keeps reiterating that you must set aside a certain number of hours each day to write. I get that - it's just the reality of it that kills me. I think women are conditioned to put themselves last - or at least mothers are.
Food for thought.