With cars it’s relatively easy to determine if a particular collision occurred before or after you bought insurance. It’s also very hard to predict exactly when these commissions will occur. Consequently, it is not so easy to delay and only buy a policy when you already have a claim ready to go.
With many progressive diseases, it’s much easier to wait and only buy insurance if you think it’s going to be expensive, but haven’t been diagnosed with anything yet. That’s why health insurance has open enrollment periods.