Great question! I'm actually not even sure if it's less salty, but they definitely consume much much less pickles (usually a tiny portion, as a topping to rice, on a small plate), and the brine is usually made up of a variety of ingredients (sometimes including rice vinegar, sake, miso paste, shiso leaves, umeboshi, etc.)

There are so many different kinds of pickles to Japanese food, most of which rely on salt rather than vinegar, but a variety also use vinegar (called amasuzuke or suzuke). I actually don't think I can explain this question sufficiently in a comment... but this has given me an idea for a future writing piece 😊

Might not be what you're looking for, but this article is not a bad place to start if you'd like to read on different kinds of Japanese pickles!