In the US, it is explicitly wealth redistribution, from the young and healthy to the old and sick. It is still called an insurance premium because it is more politically palatable.
Because there is no guarantee that the donors now will receive the same benefits when they are older. It's almost the opposite, given demographic trends. For example, 20 to 30 years ago, getting consultation by a doctor was easier, but now you often have to get consultation from a physician assistant, if you're lucky, or more likely a nurse practitioner, who are far less qualified than doctors.
Also, ACA started in 2010, so all the people receiving the most healthcare now (age 50 to 65) had a good chunk of working years where they never paid into the system, but they received a lot of benefits. Also, there is no mandate to pay the premiums in the US, so many people don't pay into it at all, until they need the healthcare.