Aside from the obvious, that it is terribly difficult to find a job at home to pay as much as an outside job, I have a couple of other reasons.
First of all, it is truly impossible to work and watch kids at the same time. While attempting to do both, you will actually be doing each one half-assed and this isn't fair to the kids or to the job. Kids need way too much attention when they are home and if you are not giving it to them, you might as well be at an office letting someone else play with them. I think that they would benefit far more. While you are working, they are running around with little structure and little supervision.
Secondly, I think thatt it is important for moms to get out and have their own life, It makes most women happier and fulfilled. When mom is happy, everyone is happy. I think that your children appreciate their time with you and you appreciate your time with them. I give my son 150% of my attention when i get off and we have a great time. Most of the stay at home moms that I have met look terrible, their kids are terribly rude and disobedient. They have forgotten to appreciate their time together and raising kids becomes a chore, which doesn't service anyone. My son LOVES his daycare, which is actually a private school with a fantastic staff. He gets excited every morning and laughs when his teacher comes to the door to get him. He loves the kids and is crazy social for being 8 months old. On top of that, he has learned so incredibly much from being around other babies.