Going on the tip of the iceberg theory on how many get caught, there's a shockingly large number of folks making decent money at the bulge bracket banks willing to take the risk on insider trading to not make much more than they were getting in bonuses. Heck, even if the ones getting caught are all of them, there's a shockingly large number! It would be interesting to see a behavioral psychology analysis as to why.
The relentless pursuit of an edge to earn more. More often than not, they create ingenious ideas. Othertimes they cross the lines of legality and/or morality. Renaissance Technologies came up with a tax structure to turn short term stat arb trading into long term tax gains. Enron came up with some fancy accounting structures too...