Some people have said some pretty insensitive things here. Let me tell you my side, and you'll see why I feel this way.
Someone said that SAHM can afford to do so because they are financially affluent. Yes, they have to be extremely financially affluent or must sacrifice a certain quality of life.
Personally, I hold a bachelor's degree and my husband has a master's. We are both educators. We make a pretty good living as professionals. We have two kids and we own a house (well, we will in about 27 years!). We have two cars, one is paid for and the other is leased. But we have student loan debt, and we have credit card debt from the years that we had to make it on one income or no income due to college (we could not both afford to go at the same time). I HAVE to work for us to even be able to eat and pay off our debt. We would continue to go deeper into debt if only one of us worked while the other stayed home with the kids. Sure, maybe the solution is to wait and have kids. Well, let me tell you. I waited until 28 years of age. Much later, and I would not be able to have the number of children we wanted safely. Risks to babies increase as mom's age increases. It's a choice we made, and let me tell you, we thought about it extensively - serious planned parenthood. Not like all the people out there who just get pregnant willy nilly. And trust me, I am a FANTASTIC parent. I may not spend every living moment with my children playing with them, teaching them, just like I'm sure SAHM's do (yeah, right!). But I spend serious quality time with my kids when I am with them. And I am constantly teaching them - everything I can, not just school stuff, but life stuff, too. And my kids are smart. My 4 year old can explain things that some adults can't even figure out. And his childcare providers all along have always commented on how he knew everything they were going to teach him - because I teach him at home! I am my child's first parent.
So in conclusion, you might say that I agree with your question. Yes, in my experience, my children have benefited greatly from preschool as opposed to my staying at home to raise them. Without my job (which means I have to send my kids to preschool), we would either not be able to afford food for our family to eat or we would not have children at all. Kids can't develop without food, and they can't develop if they're never born.