S.C.
I've actually been made to feel guilty by other moms before because I never put my kids in a pre-school. I'm a SAHM mom! Why on earth should I send them away? But I was very much like you....surprised to learn one day that it's completely the norm. Even my pediatrician started asking at every visit if they were in pre-school (which, in her mind, meant they were getting proper "socialization"). All I could do was stutter and stammer about playdates and the whole gaggle of cousins their age that live right down the street.
I have a feeling that, in spite of what they say, most people put their kids in pre-school because they want a break.