I just think life works out differently for people.
I'm in touch via social media with a few people from school but not close. My longest term friends are two of my closest friends from uni. We were a 'foursome' but the 4th dropped off the radar a few years after uni. The two remaining friends have stuck together through thick and thin for 40 years or more. We don't see each other because we all live in different countries, one on the other side of the world, but when we do meet up, the years just fall away.
I have 2 friends from a group of 6 on a postgrad I did a couple of years after uni. Lost touch with three of them, and sadly one took her life a number of years back. Again most of our contact is via SM, and one lives in another country, but same thing as the others when we do meet up.
I made another lovely friend over 30 years ago on another postgrad, and we don't live that far from each other and we meet up regularly.
Not sure how many is considered a 'friendship group'? Through kids' primary school, I made friends with 4 mums (none of our kids were actually the same age funnily enough!) and we went on nights out quite often and weekends away. All good, we had a great laugh. However, following a weekend away, two of us were suddenly ghosted by the other two - no fallout, no obvious reason. I think it may have been that this was when it became apparent that the two of them wanted to spend their time in pubs chatting up men, while me and my other friend wanted to explore the place we'd travelled to.
My remaining friend is such a lovely person, and she included me with her friends she has had for years, two of them since childhood. We're all good friends now and they're all lovely and with none of the drama that surrounded the two who dropped us.
I consider myself very lucky.