[Dalewyn]

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.

If you just want a TL;DR, here: People are shit.

[schmidtleonard]

The government can do #1 just fine.

[Dalewyn]

You greatly underestimate the sheer effort undertaken by most people to not pay Social Security tax.