So they dont fill with water when it rains, or gravel following a kitty litter excursion. Probably. Or 'fashion'.
Most are Dainese are they not ? Their leathers are designed to go over the boots in most of the new designs. See Guy Martin's as well.
Never noticed but prob more aerodynamic Or so less chance of catching on sticky out bits when moving about the bike
I know the Daniele are, not ally noticed other makes have but could be what has been suggested already