Short answer
Because the headline is measured with the screen asleep between glances. Where makers publish both figures, turning the always-on display on leaves a median 39% of the headline in this catalog, and a GPS workout drains faster still.
Each maker measures its headline battery figure under its own test script. Apple publishes its script: the 'all-day' figure assumes 300 time checks, 90 notifications, 15 minutes of app use and a 60-minute workout with music over 18 hours. Other makers describe 'typical use' less precisely. None of these scripts is your day.
Most makers also publish a figure with the always-on display enabled. 21 of the 53 watches here have one; 23 of the 44 with an OLED or AMOLED screen do not, so for those nobody outside the maker knows the always-on figure.
Across the 21 watches with both figures, the always-on figure runs from 25% of the headline (the Garmin Venu X1) to 100% (the Google Pixel Watch 4). The median is 39%.
GPS drains faster than either. The longest GPS figure here is 3 days on the Garmin Enduro 4; the app-platform smartwatches quote a few hours to a day.
Compare watches on the figure that matches how you will wear them: the always-on figure if you want the screen readable at a glance, the GPS figure if you track long workouts. Every watch page here shows all three, and says 'not published' where the maker gives none.
Maker claims quoted on this page are the maker’s own and are labelled as such. This page is general information, not medical advice.
Fitness · Garmin
Best for: Someone who wants maps on a large, thin screen and prefers a rectangular watch
Fitness · Garmin
Best for: A health-focused iPhone or Android user who wants an Apple Watch-like design with a week-plus battery
Everyday smartwatch · Wear OS
Best for: Android owners who want Pixel Watch health features for less and don't need the newest software
Every watch here needs a phone to set up and to sync. Away from it, 9 have an LTE version for calls and messages, 28 store music, and 6 combine LTE, music and contactless payments.
No watch here needs a subscription to work. 41 of 53 come from makers that sell an optional paid tier; what it gates ranges from video classes to coaching and deeper analysis.
Every specification explained, with the real numbers from the catalog behind each one.