> What I've seen happen is other parents socialize with other parents with kids of a similar age. Two moms who both have babies visit each other and hang out, etc.

That does not make isolation choice. That makes some people finding friends despite suddenly seriously limited pool of options. You are starting from zero, not knowing anyone and loosing both actual existing friends and actual related hobbies. The only thing you have in common with most of those people is the similar age kid.

You need to be seriously extroverted to be able to create friendship out of nothing after you have seen each other twice on playground while also supervising the toddler. People here complain about loneliness while having incredibly easy situation compared to that, seriously.