@MMmomDD
There certainly are features of the school environment which inhibit social opportunities. School wasn't explicitly designed to promote socialising (were you never told "we are not here to socialise, young lady" when you tried to talk to your friends?). Ofsted doesn't particularly reward schools for providing good social opportunities. That shows.
When do kids at school have the chance to simply hang out and play and chat for several hours uninterrupted? That leisurely approach can be key for shifting from "acquaintance" to "friend". At eight, my older child rarely had playdates which lasted less than four hours. That's time enough to play a bit of football, discover you've had enough of football, flop on the sofa for some telly and see whether you might like some of the same programmes, blow up a load of balloons and bat them around like crazy, look at the comics your friend drew, and get around to showing your guest your Minecraft world even though they didn't think they liked Minecraft, and for them to realise that they actually do like Minecraft now they've been introduced to a different way of playing it.
It isn't just the lack of time that's an issue with school-based socialising. They don't have access to all the stuff they might want to play with and the environment in which to do it. In this case, maybe only football is possible. Minecraft is definitely not something kids can play together in most primary schools, so how would they bond over it?
Then there's the lack of privacy. Relationships are carried out in the spotlight. It's hard for kids to relax and be genuine if they know their peers are looking on and judging everything they do. Big kids don't bring their cuddly toys to school to play with. Even if they've somehow discovered a couple of others who also like to play with cuddly toys (unlikely: no one would admit to that), what will everyone else say?
What if you enjoy the company of people who are older or younger than you are? In some schools that is literally impossible. How can a Y6 child play with older kids when they don't even go to the same school? Even if you are physically in the same place with kids of a different age, you or the other kid probably don't dare to associate for fear of being teased for playing with babies.
Why the teasing? Well, if you're stuck in the same place exclusively with the same 30 people for 30 hours a week, you can't risk ostracism. You learn to conform. Maybe you even join in teasing other kids so you can gain social standing yourself, because this class is your entire social world. The price of being yourself is often too high. I bet a proportion of the "football-mad" kids in the OP's son's class do not actually love football, but they play it because it feels like their only option.
When my social butterfly tried school at the age of nine, I expected her to love the social side of it. She did, but only for one single day. Then she began to discover all of the above problems. She made the best of it and stuck it out for the full term we had agreed. Knowing there was another world outside of school to which she would soon return, able not to care too much what anyone thought, she played football with the boys (taboo at that school), refused to declare a best friend in the class ("but EVERYONE has to have a best friend!") and associated with kids in a younger year group (downright shocking, apparently). Then she left, preferring the better social opportunities on offer outside of school.