That's a concept, it's known as cafeteria coverage. The reality is the difference is only about 1%.
The reason being, is that having an infinite number of custom coverage policies also costs money, so the insurer can save time and money by offering one plan with the large list of coverages. It also prevents the sale of policies that cover almost nothing.
Additionally, it allows "cross subsidization" where the coverages you want will cost less in the package than individually. You are subsidizing the maternity for a young couple, and in return they are subsidizing your knee replacement.
The large pool of otherwise healthy individuals makes that all work.
What really throws the monkey wrench into the works is the government mandate "Guaranteed Issue". This forces insurers to accept new applicants who already have cancer or AIDS, (or something else very expensive) and place them in the same pool as healthy people.
Since premiums are directly tied to claims...guess what happens to premiums?
Oh well.. the best minds in America worked on this monstrosity for 3 years.. It is up to us to survive it or die trying. ;-}
talk about a tainted genepool tho.. one size does not fit all.