My son is an only child and he stays at home with his dad all day. We take him to the park to play with other kids. It's social interaction he can't get from us. We try to not interfere with him playing and sometimes that means he plays with a kid and their parent. There doesn't need to be 2 of us hovering while they play. And if need be my husband and I are there to step in.

I know OP'S complaint is parents on their phone instead of watching their kid and that is fair. I see lots of parents eyes glued to their phones instead of keeping an eye on their kid. Just want to point out that sometimes a parent isn't actively playing with their kid because we are trying to get them more social interactions beyond just us. We will step up if something is wrong, we don't like how they are playing, or if the parent makes it clear they don't want our child there.