1) Age Rating Factors
https://www.cms.gov/CCIIO/Programs-and-Initiatives/Health-In...
2) Out of pocket maximums for in network care
3) Elimination of pre-existing conditions, i.e. accepting all lives no matter how costly
4) The recently eliminated mandate to purchase health insurance.
The above conditions mean that premiums from healthy people that don't use healthcare is used to pay for the healthcare for sick people. It gets a little complicated with the ability for employers to silo their employees' lives from the rest of the population, but in general, US health insurance premiums are basically a tax on the healthy to pay for the healthcare for the sick.