Just think of a payoff: They'll pay you a million dollars for X information. You get caught, go to prison for 20 years at least. That's only $50,000 a year. You could have made a lot more than that as a cleared admin, and avoided a romantic relationship with Bubba.
In reality, they don't usually pay that much for a run-of-the-mill information passer. Jonathan Pollard got $1,500 a month from the Israelis, and got life in prison. Robert Hanssen was a very high level spy, not just an admin, so he got $1.4 million over 22 years, and the rest of his life in prison (where he will die).
And if you think you're so smart that you have a very low chance of getting caught, then you're an idiot. Hanssen himself was a counterintelligence agent, and that helped him go for as long as he did, but he still got caught.
BTW, one of the things they check is unaccounted indicators of wealth, and they do ask friends and neighbors, and check your financials. I remember a new soldier was investigated back in the 80s because he showed up one day with a new BMW 7-series. This wasn't even caught during a reinvestigation, they just noticed. Turns out dad was rich and gave him the car as a reward for joining the Army. With such a clear reason he was okay, but had he not been able to show a solid source for the money he would have been in a whole heap of trouble.