I suspect that the USA requires a passport book for international departing passengers primarily for an ICAO-standardized method of verifying identity and citizenship.

Here's another example: EU citizens can enter and depart the EU on their national identity cards issued by any EU country. However, in order to travel from the USA to the EU, they must show a passport to the carrier providing the transportation.

I have traveled US-EU with EU citizens who have to show their passport issued by Spain or Germany in order to board the EU-bound flight, but then just flash their national ID card to the immigration officer upon arrival. No stamp, no swipe, nothing. They just flash their card and keep walking. So, one could ask, why would an airline require a passport to travel to a country that did not require a passport for entry?!

I suspect the answer is that there are just too many different regulations in place around the world for travelers to enter certain countries and/or regions, and it would be crazy to expect airlines to administer those regulations correctly. (Furthermore, what if an international flight diverts to a third country where an ID card is not sufficient for entry for certain citizenships) So, it is just easier to say, PASSPORT REQUIRED, period.

In fact, I believe there are a number of Caribbean island nations that allow US Citizens to enter by air using just a passport card. But they wouldn't be allowed to board their flight to travel there or return unless they could show the airline their passport book. Go figure!