Why is the battery life on an Apple Watch only hours compared to days for other devices?

I do not understand why. What’s going on on the background that is draining so much battery that it cannot last for many days like on other devices?

My wife’s Fitbit Sense which also has a similar display lasts 3 or 4 days connected to her android phone. It has a constant heart rate monitor unlike the Apple Watch that only takes readings every 10 minutes or so, pushes app notifications, and receives calls and texts on the watch itself, and analyzes advanced sleep patterns which the Apple Watch does not with native apps.

Why is the battery so poor?