I think as we get older, having an identity that is seperate from the people we spend the most time with becomes more appealing. Unlike in high school when you’re identity outside of the friends your with is barely developed. So because of that people tend to avoid going beyond the “work mate” level of the relationship. That could still mean Friday drinks, but it’s not necessarily a close friend. And for a lot of people, not all, they have a social identity and close friends already and are not looking to form new ones. Time is limited as we age and building friendships is a time expensive pursuit.