It’s not - there are tons of places with no access to swimming. It also has less to do with urban/suburban. If anything, urban is likely to get more, as they can potentially reach public pools.

It’s about access to water. If you parents don’t take you, if there are none around to naturally discover, if there are none around which you personally have access to, you don’t learn to swim.

If you do, you typically do. Simple as. I really do not understand why people seem to think everyone has a pool. You can tell many of the people in this sub grew up middle class at least!