My Apple Watch alarm stops vibrating/sounding by itself but it stays on the “snooze/stop” screen

so that I don’t wake up my wife or new baby, I set my alarm on vibrate on my watch. It gets me up a lot of the time but if it’s a rough night and I’m particularly difficult to wake up, then I sleep right through the alarm and I wake up about an hour later on my own and my watch has the screen where I can snooze or turn off the alarm. So I know that the alarm initially goes off, but it stops vibrating at some point and just stays on that screen. Why doesn’t it just keep going until I turn it off?