Many of them do not reciprocate with an invite in return. A few have invited my DD to their houses but only once.
Is your daughter upset or worried about this issue at all? If not, I wouldn't worry about it or give it any further thought. My son is only 3, and the kids from daycare that he is friends with and does activities with outside of school are kids that he selected as friends. I'm not necessarily friends with any of their parents. It would be convenient if I were, but basically, I am just polite and pleasent to these people for purposes of facilitating the interaction between our kids.