An important factor is family. First, we're expected to retreat into our nuclear family (marriage and children), so that friends - be they high school buddy, college chums or workmates - become secondary. Second, workmates who do marry and have kids, bond over that shared experience. They have a natural conversation piece (spouse, children, the local schools, burden of changing diapers, the travails of kids playing sports, the stresses of kids applying to colleges,...).
The consequences are clear. Because family come first, friends - a work or otherwise - become demoted to the B-team, once young-adults in the workplace form families. And once the kids arrive, the parents' thinking naturally shifts, so that to the extent that they have mental-space available for friendships, those friendships tend to revolve around parenting.
Example: Smith is the consummate frat-boy in college. Parties, locker-room towel-slapping etc. Smith graduates and joins a bunch of folks in the same cohort at Megacorp. For a couple of years, they all go drinking together, take ski trips together, play golf together. But then one after another they marry. Then they kids arrive. By 30, most of the fellows in Smith's circle at Megacorp are in the suburbs. They bought minivans. They talk about teething, cribs, first words and first steps. They're still cordial with Smith... that "propinquity" thing... but it's clear that their common-core has diminished.
