The equipment usually isn't sized for adults. I barely fit into the swing set at my local park, and I'm smaller than average. Even though I can technically use it, it's very uncomfortable.
But besides that, as a grown man I'm going to get some very weird looks from parents if I waltz up to the jungle gym and start climbing with their kids. There may not be a law against it, but it's certainly not socially acceptable.
According to knowyourmeme, this is a photo from a 2018 session of Japanese Parliament. The guy in the center is calling for a vote on a controversial bill, and a bunch of opposition party members are attempting to physically prevent him from doing so. Apparently this is a thing they do in the Japanese Parliament sometimes?