Motorcycle bars, specifically motocross ones are a mere 22.2mm, but they have a cross brace which allows for more strength than diameter alone would suggest. Given that mountain bikers tend to like low rise bars, and balk at weight, cross braces just aren’t an option, hence 25.4 and now 31.8. Also remember that while 31.8 bars might be heavier than 25.4, they will also be wider and stronger, and that’s a trade off most people are willing to make, especially now that we can have carbon bars.
I wish that 12mm rear axles were on every bike, just for the peace of mind rather than any nebulous stiffness arguments.