Ok, I think there is (can be) a weird school gate thing.
If you work in a big organisation, or otherwise have good and inclusive social skills, you just expect a group that is thrown together (by reason of something random like having children in the same class) to be on workable, however superficial, nodding / chatting terms. (In all honestly I personally would prefer to be able to time our arrival precisely enough that this isn't necessary, or at least choose to avoid it sometimes, but somehow I can't manage the timing right and we are often early and dcs like that because they play with their friends.)
If you have slightly less well developed social skills that are based around searching for friends and people who have certain things in common with you - and, crucially - not caring how obviously you ignore the others - then you will not necessarily be expecting to nod and chat to just anyone. You'll have a "saving seats" mentality (there are not literally seats of course, but when adults in groups like choirs or work places have special chums that they have to sit next to it makes me roll my eyes, especially when the person actually tells someone else they may not sit there.) (I have never been in a work place where that would be acceptable - all piling into a big meeting and someone saying when you go to sit down "oh sorry, no, that's where Jane is going to sit when she gets here" - but I have seen teachers do this - Jesus, is there something about SCHOOLS in particular?)
I honestly think that it's just poor social skills. We all have people we prefer, and we can have coffee with them or go to the pub with them, but some people are crap at making the others feel halfway comfortable in situations where we all have no choice but to be there, and that's just being a bit rubbish really. Not death penalty rubbish; not as strong as bitchy, or mean; just... not being great at managing social dynamics, or not caring.
In some communities there are such strong positive personalities that work towards inclusion that this doesn't happen. there are some people that are so charismatic and so sincerely inclusive that they make everyone else aware of how it's a bit shit to be otherwise. They sort of glamorise inclusion.
these people are special and not every community has one. (I wish I was one but I am not! I am merely a functional small-talker rather than a charismatic social force of cohesion)
Now on threads like this you often get a lot of defensive posts saying that the phenomenon doesn't exist. It just does, not necessarily everywhere, but it just does. I find the defensiveness interesting. What does it mean?
To be clear:
it doesn't bother me that some women are friends with each other and not me.
I do not particularly expect to make friends at my dcs' school (not that I am prejudiced against that idea either, but you know, it's not what I deeply hope for);
It is probably the case that there a lot of parents that I don't have that much in common with, probably, because we're all different, so, yeah that's a thing;
I don't worry about it because I don't know the women well enough to worry that I have offended them or done something wrong, so that's not something that preys on my mind;
The social discomfort is not a huge worry in my life, something that makes me sad or that feels like a huge slight or that keeps me awake at night.
BUT.
It is there. It is uncomfortable to go, a few days a week, without a "cover task" that can make you look busy, to a place where you are going to stand about in the presence of people who all know each other by sight and many by name, and some of them are going to pretty much blank you. It's not the worst thing in the world. but why the fuck won't people just grow the hell up and say hello?
Because they have poor social skills, and are a bit childish. They don't have the work experience or the social experience or the general emotional intelligence and nous to just behave a bit better.