And because in the US, there is no prohibition on mixing domestic and international departures from the same terminal, with "sterile" areas existing only for international arrivals, the burden is placed on the gate agents rather than check-point security to confirm proper travel documents prior to boarding.

At SFO, it's possible to reach the boarding gates at international G terminal through any one of three security checkpoints in the domestic Terminal 3. So it would simply be impossible to conduct passport checks at the security check-points.