It's not about whether they're smart or not - it's that a multimillionaire can lose a bunch of money on a private company and not be out their retirement savings. That's not nearly as true for the typical middle class person.
I'm sure there are some people who could safely drive on the roads without speed limits and other traffic laws, but the number of people who could not is far greater. So it is with careful, responsible investing. Many laws are designed to work on a statistical basis, for the greater overall good. This is one of them.
It's not designed to prevent 'ordinary' people from sharing in investment profits, it's designed to protect the public purse from having to support millions of people that lost their savings due to unwise investments.
