Many reasons really, it all comes down to risk assessment. Cardiac arrest is the highest cause of death in the UK - risk 10/10, without the timely application of such equipment, the worst outcome is almost certain - so severity 10/10, with the early use of such equipment the likelihood of success is reasonably high - so mitigation 8/10 - as Chris says, no brainer.
Then there's the fact that our nearest public one is a mile and a half way down a single track country road. Also, our nearest neighbours (200 m and 500m away) are all in their eighties. And, we're not getting any younger!
The cost? less than a mid range mountain bike, about the price of a new iPhone, less than the cost of a quality family tent. My point being, we all spend this kind of money on all sorts of things but few spend it on staying alive