Probably use-case. I only really engage with my watch to check the time and my pulse which I can do with a glance, otherwise the screen is permanently off. The other times are when I'm doing some activity (running, swimming) and GPS is on but for example with running the screen still only shows when I glance at it. Apple still has an enormous amount of room to optimise their hardware and software in this space but I've no idea why they haven't done it yet.