When you compare the USA to many overseas airports, yes, it does seem that we are nowhere near as streamlined with International-to-International connections as, say, HKG or SIN or NRT, or anywhere in the Schengen zone. I believe it has to do with a couple of additional things (in addition to mixed domestic/int'l departing pax in the same terminal and the lack of exit border controls in the USA):
The USA is a big country, with lots of domestic flights that can be shorthaul or longhaul. Most major US airports have a substantial number (even vast majority) of domestic connecting passengers, as already noted.
The same thing cannot be said for most of the rest of the world's major airports. HKG/SIN, two of the world's busiest airports, have ZERO domestic flights. Every single passenger transferring at either of these airports is an International-to-International passenger. EVERY flight is an international flight.
Japan, the UK, and most Schengen countries are small (even tiny) by comparison to the USA, with many FEWER domestic flights. So, most connecting passengers are International-to-International, and they can be kept isolated in a sterile part of the airport and not need to be processed into the transfer country. And, it is easy to separate any actual domestic flights into their own terminal. Schengen airports have built separate terminals or portions of terminals for intra-Schengen flights that are technically international but do not involve passport control. Again, the countries that house most of the world's major airports are SMALL countries (geographically speaking). Of course, there are exceptions such as Russia and China and Australia. But I'm generalizing to make a point.
2) Most air travelers in the USA are domestic air travelers. Again, the USA is a big country. The vast majority of leisure and business travelers do not fly internationally anywhere near as often as leisure or business travelers who are based at HKG or SIN or LHR or NRT. American travelers don't have to fly internationally. They can have a beach or ski vacation, or cruise to Alaska or vacation in Hawaii, or go to Miami, or ice fishing in Maine, all without ever crossing an international border. The USA has cities where the economy is based on finance, others based on tourism, others based on entertainment (LAS), others based on industry. In short, you can find basically anything the average person might want, without leaving the country. So, most Americans choose not to travel internationally. In fact, the figures are staggering....only
36% of eligible U.S. citizens have been issued passports!
(Compare that to the average middle-class Londoner, Hong Konger or Singaporean.....he or she might leave the country two or three times a month, just for leisure/weekend getaways! It is not uncommon for business travelers in Europe to attend meetings in 3-4 countries in the course of 24-48 hours!)