It's probably because open water swimming is rather a niche activity in the UK, not something that 'everyone' does.
I think there's a lot of truth in this.
If there's nowhere outdoors where they're encouraged to swim, then people won't get the experience of swimming in anything other than a pool or warm sea.
I'm in Maine and pretty much every town has a beach - sea, lake or river. There are cordoned off areas where boats can't go, rafts to jump off, kayaks to rent, a portapotty/toilets and sometimes a changing room (although usually not).
We have an 11 week long school summer holiday and most kids will swim a lot - either with summer camp (DD2 is at Sebago Lake today with her camp) or with their parents or as they get older just with friends.
Occasionally people will still drown though - I mentioned the boy who drowned last week who was at a state park lake beach with a lifeguard, in the cordoned off area, surrounded by dozens of friends and teachers, and drowned.