I don't know if it's always more than 50% but on U.S. domestic flights a lot of people are carry-on only. It's far more than half on the routes and days frequented by business travelers. On routes and days where more consumer, family and vacation travelers fly it may not quite be half but it's still close. Personally, I haven't checked a bag in over ten years. Using packing cubes it's possible to fit a huge amount in a well-designed modern suitcase.

The U.S. is different in this way from many other regions, especially much of the EU. There are specific reasons I've noticed:

- Due to the shorter EU domestic routes, it's more common to see smaller aircraft with much less overhead space for bags.

- For EU domestic routes, limits on carry-on bag size / count tend to be lower, more frequent and enforced more stringently (even when the aircraft in use isn't space-limited).

- In many countries there are different carry-on bag size / count allowances between domestic and long-haul international. In the U.S. almost all domestic flights use the larger international allowances (the rare exceptions usually being 'puddle-jumper' connections).

- In the U.S. checking bag compliance at the gate isn't as frequent or stringent. The nominal limit is a small suitcase + a personal item. On intra-E.U. flights, I see large backpacks rejected as the 'personal item' that are routinely accepted in the U.S. A higher percentage of U.S. passengers have maximum 22-inch roller bags than I see in the E.U. You can fit a lot in a 22" bag + large backpack.

- My perception is that elsewhere in the world, the average person on a domestic flight will be away from home longer than in the U.S. I assume this is due to the other regions often having better inter-city / region train and bus options than the U.S. which take a larger share of shorter duration trips.

- Other less significant factors might include U.S. business and evening attire being a bit more casual on average, making is easier to pack small as well as U.S. airline industry competition making shorter duration (but not necessarily shorter distance) U.S. domestic flights more accessible to more consumers. A lot of U.S. middle-class consumers now frequently use flights for weekend trips over 1,000 mi away. The U.S. has a larger number of smaller commercial airports in second -tier cities that are still fairly easy to get through quickly, even with TSA security. This can make same-day jet trips to cities ~500 to ~1000 mi away not much more involved than a typical EU train trip to a nearby city. For about a year, I did same-day and overnight jet flights from San Diego to Sacramento (~900 mi / 80 mins) about twice a week, often with nothing but a bike messenger bag as carry-on. I know a guy that did San Jose - Burbank as a daily commute for several months. A larger airport like SFO or LAX can add nearly an hour on each end just due to airport logistics and location but 2nd tier airports with longer direct flights make it possible. I think that's more unusual in other countries.