There is a huge survival bias here: the ones we know who did it are unsophisticated about it so were easy to catch. The ones we suspect might be doing it are sophisticated enough that there is no evidence of their wrong doing. Maybe they aren't doing it, or maybe they are just good at doing it.

One of the ways hedge funds do, or used to, do is this strategy: trade a ticker very regularly (every other day, or thrice a week), then get the insider info, make the profit in that week. Then buy and sell. A month later, move on to another ticker.