Please don‘t extrapolate from the US healthcare system to insurances in general. Insurances collect money from many so in the case something happens to an individual that individual doesn‘t need take the full financial loss. This makes a lot of sense, because it would very inefficient if everyone would save money in order to pay for a potential cancer treatment. Cancer is rare, but in aggregate it is just small amount each month.
The job of the insurance is to define that monthly amount (which is not trivial to do), collect it, store it and eventually pay it out.
On another note, unless an insurance is mandatory you can usually opt to pay yourself.
