I assure you, it is only possible to avoid risk if you're a small fish and extremely careful. For instance, you have a good chance of getting away with skimming a neighborhood stock fund you setup. If you are managing or receiving millions of dollars in investment, you can bet anything fishy will show up on the SECs radar at some point. Most people with large sums of money don't get where they are by being stupid with it, and if they are the victims of fraud, they know where to go. While the SEC can't bring criminal charges, they have top notch investigators. Once they smell the fish, they won't stop until the entire scheme is unraveled.
I can't speak directly to the observation you made, but I personally know a high up SEC investigator, and he obsesses about the most minute details of the cases he works on and lives to take on the toughest cases possible. My guess is distribution of cases the SEC prosecutes are very heavy tailed. There are many dumb cases that are very easy to find and prosecute that involve 10s to 100s of thousands of dollars. Think skimming, ponzi schemes etc. The more sophisticated and larger cases are also more risky (for the fraudster), harder to execute, and much more rare. That might explain your observations.