[nmstoker]

I think you may be missing the point: preventative treatment is typically much less expensive, for instance behaviour and dietary changes do not require drugs at all and avoiding some conditions can be helped by drugs which have long since come off patents.

But even with your point, all insurance companies I've ever had cover with in the UK have had some element of support for preventing illness (periodic assessments, support material and trackers) and, at least with people covered under company schemes, they clearly have an incentive to offer more if you are at risk of becoming affected by a preventable illness.