Because that is what the SEC was created for, and (in theory) it is their job to protect regular invesors from market risks. Now how effectively that works is a different conversation, but at the very least you have reporting requirements, earnings releases, material disclosures, insider trading laws, SIPC insurance, circuit breakers etc. It is very unlikely that you are going to lose all your money in a stable blue chip company or broad index fund, but a regular joe trying to invest in a hot "private investing opportunity" is absolutely going to be taken for a ride.