Because some of those moderately healthy people will still suffer from disease or injury that will incur large healthcare costs. The whole point of insurance is that it spreads those costs around. While you may be lucky enough to not find that you have a congenital heart defect that costs $100,000 in surgery to correct, your premium helps pay for that one guy out of 100,000 that does. And it means that the public doesn't have to pay your healthcare costs if you do suffer from an illness that carries catastrophic healthcare costs.
Spot on. And I want to stress the congenital part there—a lot of congenital conditions go undetected until a person hits their late 20s or early 30s, just because they were otherwise healthy and asymptomatic. So just after they've been trained or educated and they're entering their prime work and childrearing years, a previously hidden health condition catches up with them and saddles them with unbearable financial burdens for the rest of their lives.
Anybody who thinks they are one of these proverbial "young healthy people who don't need insurance" doesn't really know it. And basically, they're choosing to opt out of the only sort of system that could protect them from that.