Lots of people on MN are deeply weird when it comes to all matters friend or social skills related. People who don't answer the door, expect people to text in advance of a phone call, who have no friends, don't want friends, view friendship as tit-for-tat transactions - it's never ending.
There are also hundreds of threads about "mean girls" at school gates and it becomes a self fulfilling prophecy. If you've already got hang ups around friendship and being excluded from groups (whether real or imagined), then when your child starts school you are going to be hyper vigilant for any perceived slight. Someone who talks to you one day and ignores you the next isn't just having a bad day or has forgotten to put her contact lenses in, she is a spiteful cliquey bitch who is deliberately going out of her way to be nasty. Three women chatting together aren't people who know each other from yoga or because they have older children, they are spiteful bitches who are going out of their way to be nasty.
You get the picture. I have had three kids go through various nurseries and schools and mixed with hundreds of parents at school gates. Never witnessed any of this but then I was never actively looking for it. I never thought of the school gates as anywhere to forge valuable, lasting friendships. It's a short period of time, twice a day, where adults are rushing off to do other things, wrangling younger siblings and only have time to acknowledge in passing people they already know rather than actively seeking out people standing alone.
Making friends with other parents in your child's class is definitely do-able, but you have to put yourself out there, attend the school events, step up and help out. It's hard work. This idea that an entire social circle should miraculously fall into place by picking your child up from school 5 times a week is just crazy.