Neither of mine have been invited to parties of children from childcare (they went two days a week). But we didn't have any parties where we have kids from childcare either so I didn't think much of it. I think at that age unless they are seeing kids outside childcare on playdates then it's completely normal not to invite them.
We haven't had a party before and I never really thought much about it but like you I didn't really think people would invite children from daycare unless saw them at other times - but the friend from MG's DD doesn't see anyone from CC outside of CC either - so seems does happen. Who knows why DD has never got one invite - she wants to invite several friends from CC this year so if it is reciprocal then maybe she will start getting invites - and if no one comes maybe there is some other reason. DD isn't bothered she knows there are parties going on but she has decided she has been on holiday for all of them :-)