Basic economics, actually.
Some commodities are terribly resistent to price change based on a pure supply/demand equilibrium curve.
This is reffered to as "inelasticity." something that responds a LOT to price changes is said to be "elastic." For example, amoung adults, raising the price of a pack of smokes (even exorbitantly) produces almost no change in the demand for cigarettes. Thus, for the adult market, we'd say that demand is "inelastic." But amoung teenagers, who typically have little money, and are thus extremely sensitive to changes in price, we'd say that demand for smokes amoung teenagers is "highly elastic."
Likewise, gasoline, in the short and medium terms, demand is almost completely inelastic, that is: it changes almost not at all when price goes up, because it's necessary.
And yes, health insurance, because it's considered necessary, and in many places, mandated by law, is fairly inelastic in its demand relative to price. They raise prices, we keep buying it. And thus knowing they could do so once, they will continue to raaise price, because health insurance, given the place it occupies in society (called "externalities" in economics) is largely immune to much of pure supply and demand forces.
There are too many other mitigating factors.
Of course, from a realistic legal perspective, the only thing realtively sure to slow the growth in heath insurance cost is tort reform, that is: limiting the amount, in dollars, any one doctor can be sued for any one accident/mistake/phuckup.