Earlier quoted context omitted.
No, that's because in the US they're handling the international flights separately. It's also the reason why even when you have a layover, you need to clear customs. Domestic flights in the US are like busses/trains elsewhere. Most people fly without a checked bag
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Absolutely true and you can tell at least in the old days when you'd fly southwest. Every other airline the overhead bin fills up. It is an inevitable drama when the flight attendants have to say "overheads are full now we are gate checking bags."
Southwest, at least before they changed their bag policy, would let you fly with two free checked bags. Finally everything worked as intended and those overheads were seldom used. Maybe for a jacket or purse or something, but no one was shoving a roller bag up there.
Spirit was another airline with ample overhead space, because they charged you nearly the same rate for overheads as checked bags.