Only because it needs the battery to do the litany of other things it does. As for your "or it senses a heart rate reading that's greater" question... that come

Only because it needs the battery to do the litany of other things it does. As for your "or it senses a heart rate reading that's greater" question... that comes from my actual use of the watch. I sometimes forget to put the watch into Workout mode. If I go into the Health app afterwards, I can see that the frequency of measurement goes up when the watch detects that my heart rate is above my resting heart rate. It goes from being sporadic to being continuous until it detects it go back down. So it's not that it can't do it, it just doesn't do it to conserve battery life. The Fitbits don't really have much else going on that requires the battery so they can run continuously all day. Additionally, according to most medical uses and reviews of the Apple Watch, it's more accurate than many stand-alone heart rate monitors so, as battery life improves, the watch can only go up from here.