I also really like that you can find groups of people doing the same thing you like to do, no matter how niche it is.

You can find mono-culture in both cities and rural areas. In cities there are more options, but in rural areas you can really get into it. In cities you can select from several different mono-cultures and ignore all others. In rural areas you can really get deep though because a commune can ensure you never encounter anything else, while in cities you still see them on the streets you just don't interact (though my experience is communes have outsiders come in for various supplies from time to time). If you don't live in a commune though rural areas don't provide choice - your neighbor is the only option no matter how strange they are.

That is what University is for ;)

I don't think they need that at K->grade school level (or younger) where they are first realizing how big the world actually is by exploring safely. You start with your room, your house, your yard, then spiral out until you gain confidence. Hard to do that in a dense urban area by yourself.