Learning to swim can save yours or someone elses life.
Swimming is a healthy hobby, teens can socialise together at a swimming pool.
Once you've learn't to swim you don't forget, no one can take this gift away from you it doesn't break or wear out or lose it's value whilst stuck in a cupboard.
Some people like their kids to have very nice cars or fantastic holidays or expensive clothes/shoes or a mountain of new toys at Christmas or a TV and dvd player in their bedroom and a mobile 'phone or laptop at the age of 7.
My personal opinion is that swimming is more important than consumer goods and I'd rather spend the money on swimming lessons for the dc, a useful gift that keeps its value.
My dc didn't take to it like fish, I have spent many years cheerfuly and willingly giving up my freetime taking them to weekly lessons, but they stuck at it and now happily accept swimming party invitations without worrying that they can't actually swim yet.
I value my children being able to swim, other parents may not and you're so right that there sometimes seems to be the attitude that it's school/government's job to provide lessons.