The Apple Watch feels like it's designed as a very small iPhone. It has a lot of unnecessary functionality on-device, like email, a separate iMessage client, and its own Focus settings, which can sync to the iPhone, possibly as a requirement to make the watch operate independently. All these things take up battery power.

Also, Apple sees no benefit having their watches last longer, so they're adding features instead of optimizing them, as long as it hits 18 hours or so.