It's impossible, at least given the setup of the vast majority of (if not all) US airports.
In these airports, because there is no exit control, it is possible to leave the departure lounge, without boarding your flight, and also without passing through the immigration hall. For example, if you check in to your flight, and, while you're waiting to board, you receive news of some emergency that prevents you from flying, you can just walk out past the security checkpoint and onto the street.
For a CBP officer to be certain that you've actually left the country, he would have to be certain either that you are on the plane when the doors close or that you are in the international zone of the airport. This area is normally accessible only to arriving passengers, so you would also require special handling by the airline staff in order to get on the plane. And, once you're past the special door and, by whatever route, in the plane or on the jetway, it is again possible for you to walk back up the jetway and into the departure lounge.
The CBP officer would therefore have to rely on the airline staff to assure him that you were in fact on the plane when its door closed. And, come to think of it, that's exactly what the I-94 system is supposed to achieve.
Granted, the I-94 system is quite imperfect. My wife has several entries in the database that do not have a corresponding exit.