For health insurers to be profitable, they need young healthy people to have policies and pilfer elderly sick people.
Same as any other business. If banks want to make loans, they have to have deposits that people don't want to touch. If you want to make money on the stock market, someone has to make the opposite bet against you. If you want to make money selling airline seats, you have to assume that you'll sell every one.
Assuming other people will do something (or are wrong about something) is the basis of a lot of businesses. Nearly all of them, in fact.
The difference is that banks provide economic incentives for people to deposit money. What incentive is there for a 20-something to spend $200/month for something they will probably never need?
The incentive, for me anyway, is that I can afford $200, and now I never have to worry about "what if I get cancer" or "what if I'm in a bicycle accident and need major surgery and months of physical therapy". I mean, both situations are bad, but I won't have to worry about "how will I pay for this" in addition to "how much longer am I going to be on Earth". IMO, it's money well spent. So far, my insurance company has won in terms of dollars made, but I've won in terms of peace of mind.
The incentive is that they will have coverage when they need it. Everyone will need coverage eventually because everyone gets old.
That incentive can't exist as long as insurance companies exclude people for pre-existing conditions, change rates, etc. Noone wants to pay into the system that they know is setup to not pay out.
Same as any other business. If banks want to make loans, they have to have deposits that people don't want to touch. If you want to make money on the stock market, someone has to make the opposite bet against you. If you want to make money selling airline seats, you have to assume that you'll sell every one.
Assuming other people will do something (or are wrong about something) is the basis of a lot of businesses. Nearly all of them, in fact.