The USA absolutely DOES perform departure immigration/passport checks. But it is not in the traditional sense (i.e. no checkpoint/officer at a booth), and the passenger usually is not aware it happens unless there is some issue or the record is red-flagged. When you check in for an international flight, the agent swipes your passport (or you enter details online) and the airline submits your information to DHS to perform a computer check on your record and verify that you are authorized to depart the United States, have no outstanding warrants or unpaid child support, etc. The airline also verifies that you have sufficient documentation to enter the destination country.

That is why your international boarding passes say "DOCS OK," meaning that the airline computers submitted your data to APIS and received permission to allow you to board. It is the functional equivalent of a departure immigration check, but it is performed "in the background" rather than by an officer at the airport. Nowadays, almost everything is performed electronically, and because the USA does not isolate international departing passengers, the airline is left as the primary means of permitting or denying departure from the country.

Sometimes, as noted above, CBP or ICE officers stand on the jetbridge of an outbound international flight looking for "watch list" people, but mostly they are looking for people trying to evade Customs regulations concerning (primarily) the export of undeclared currency beyond the $10,000 limit.