Why some friendships end after kids come into the picture

This assumes the friendships were established and maintained in an environment that could potentially be friendly to kids. If your friend group is based around hanging out at barbecues at each other's home, or more child-friendly environments, it will be much easier and likely to maintain those friendships VS. friendships built on generally less child-friendly activities, like long-distance cycling, weekend trips to other states/countries, bar-hopping, etc... Friendships are often built within shared social settings. Changing up the terms minimizes the very shared experience your friendship was built on

>Changing up the terms minimizes the very shared experience your friendship was built on

Yes, and that's when you find out whether the friendship was merely "built on" that experience, or is entirely composed of it.

I have lots of friendships that were formed in the kinds of experiences you describe - especially long distance cycling. Those people all have kids now, and we don't go cycle touring together. But I put the effort into finding other stuff to do with them, so we're still friends.