If you're talking about "volunteering at the PTO", my guess is that you are married with children. Most people who are married with children already have most of their social framework "set", and often times (at least in the US) so much of their free time resolves are child rearing and their childrens' activities such that most of their "neighborhood friends" revolve around the serendipitous encounters they have with other parents - the PTO as you mention, sports games, etc.
But for people at an earlier stage of their life and career, I think you'll find they often have a different outlook. They're often looking for a spouse, they have much more free time without kids so they're looking to meet people, etc.
For me personally, I'm also at a stage where my social connections are pretty set. But I got so many of my early workplace connections when I joined a job that had a "bootcamp" 2 month training program for new grads. I've gotten nearly every subsequent job over a couple decades from that first job, not to mention many close friends. I know I wouldn't have had those close connections if I had been solely WFH in my first job out of college.
The fact is that now people, across all aspects of society (not just referring to work from home) have much less opportunity for these kinds of serendipitous interactions: we can order tons of stuff online instead of shopping at retail stores, we can order food from a bajillion places instead of going out to eat, we can stay in and watch Netflix instead of going to a movie. And while all of these conveniences, individually, may be very nice, overall they have the effect that we just have a ton fewer in-person interactions, on average, with other people.