Where would they do it? At boarding? Most US airports have no wall between domestic and international flights.

Totally anecdotal, but I was once pulled aside on the jetway by a woman with a Treasury badge and a burly guy who was probably a sheriff though he never said anything. She wanted to know how much cash I was carrying. I was flying from SF to Frankfurt, and I think it was 1994. My net worth then was about $30k, so I laughed and told her I had about $200 and a credit card.

So maybe CBP doesn't check outbound travelers, but Treasury did at least once.

CBP can and does in fact carry out checks for undeclared cash exports in the jetway for international departures. Not every departure, obviously, and I don't think the selection mechanism is public, but it's a thing they're documented as doing.

I have seen them set up in the jetway after you show your ticket to the airline attendant.