That’s a good point. If it’s cold and wet I just leave the coat long. The waist belt of the pack goes around the coat.
Otherwise I’ll pull the coat up above the waistbelt and unzip it. The only negative is that it’s folded or wrinkled against my back. With a short jacket you’d still have to pull it up above the waist belt but there isn’t as much fabric being folded/wrinkled. That extra strip of fabric on the long jacket doesn’t make that much difference. Even with the coat pulled up above the waist belt, it’s longer in front which provides better rain protection.
I spend a fair amount of time without pack – day hiking from base camp or fiddling around camp in which case the longer jacket works better if it’s raining.
I was looking at jackets at REI and they all stop short of the crotch.
I bought a Gore Tex jacket at REI maybe 20 years ago and it’s longer, goes below the crotch. Maybe they used to make jackets longer? Thus my theory that the current style is to make them short. It does require less fabric so it saves some money and weight.
When I’ve made jacket longer than about 3/4 of the way from waist to knee, when I’m walking and moving my legs back and forth, the jacket rubs against the legs and is annoying.
With a pleated kilt, in the pictures I saw, it goes below the knee. Maybe with the pleats it doesn’t rub against the legs when you’re walking? That would provide better rain protection so would be a good thing.