There are two reasons, one virtuous and one sinful:
* People hate saving money, money in our hand now must be spent now. Buying High and Selling Low is in our DNA, quite literally. Nobody wants to save a buck a day for a potential externality when they can spend that buck on something right here right now. Mandatory insurance forces people to save a buck (the monthly premium) for the potential externality, forcing people to Buy Low and Sell High.
* People want wealth, some moreso than others. The secret sauce? Being the middleman has the biggest revenue compared to effort. You are literally moving money from Person A to Person C, and you as Person B can take some of it. This is textbook rent seeking.