Don't forget that in the USA the majority of business flying is domestic. The result is that there's much more of a 'turn up and climb on board' attitude (admittedly the security checks have inhibited this somewhat, but lounges don't help with that problem).
Many airports handle domestic flights only, and quite a few of those that describe themselves as 'International' do so on the basis of a once a week flight to Toronto/Tijuana
.