1. having pants with no break is a 'clean' look, more conducive to the environment where they're worn, ie the streets are ok with a sort of stacked overflowing look, which would just be tacky in a professional setting.
2. The hem sizes are different, so on narrow jeans, a bit of stacking doesn't look all that sloppy, whereas on dress pants it does.
3. There's something about the sanctity of the denim that once you've bought it, you just wear it, so even though some hemming is done, it's sort of more like you buy them as is and appreciate them for how they were meant to be worn. 4. The denim fabric itself is sort of so valued that you wouldn't want to cut any of it off and just throw it out. You enjoy every scrap.
5. Lastly, I think it's also just an aesthetic choice. Personally I think that some stacking around the ankles has a neat look. With a narrow hem it's not that over the top and it feels more runway/trendy than an office look.