I'm not sure they're _much better_ but in my experience (both as a kid and a dad) they serve different purposes. Parks and playgrounds are great, as you've mentioned (but you can also get socialisation through play groups, activity groups like swimming, school, the beach etc).

My kids do lots of stuff they couldn't do in a public space and I likely wouldn't let them do inside. They have outdoor baths in summer, grow sunflowers and herbs, collect eggs from the chickens, have 'outdoor movie nights', run around naked for hours etc. There's also social things, being in a playground or park means my kids often go and find things to do by themselves, which is great (they're great at independence). Being in our backyard forces them, most of the time, to work together to solve whatever problem they have. Those problems are often far different from what happens _inside_ the house and are just as critical to their development. Last week I saw my 2.5yo up our apricot tree throwing apricots into a bucket being held by our ~5yo because they wanted to make dessert. Could probably do that in a park, but yeah.