No insight into other countries as I’m an Aussie myself, but I can’t say enough good things about Parkruns.
One of the things I really like is that it gets “non-runners” into running. People will do it as a social thing on the weekend, and often partners/kids will join in too because it’s free, low-pressure and an approachable enough distance for most to walk/run. At the other end, they’re a great opportunity to do a 5km TT with a bit of extra motivation (my club does them frequently during winter and as part of prep for duathlons and running events), and there’s usually someone to chase all the way up to the random 14-year old guns at the front, as well as a few local pros who often use them as tempo/threshold. There’s usually a group of regular volunteers as well as some that rotate in and out, including some of the aforementioned pros which is cool.
Pretty common to see people do the local parkrun on holiday too (especially if it’s a more rural area and you can snag a CR in the absence of the actual fast people)
At a guess, I’d say our relatively nice weather and low urban density play a role- we’re lucky to have surroundings that are pleasant for outdoor activities year-round. it’s definitely possible to recreate the same experience in the middle of a big city, but IMO more difficult and less opportunities to do so. And the majority of people there are predominantly doing so for fun/social reasons, and probably less inclined to show up on a cold/rainy day than a race they’ve trained and paid for.