Different ideas for different people.

The general concept is, either it lasts for weeks or it lasts for one day, if it lasts for 3 days you'll forget to charge it, if it lasts for weeks it's okay if you forget, a small reminder can tell you to charge it enough for a day in a few minutes.

The trade off is insane in comparison, the Apple Watch is capable of a ton of things you just can't do in a longer battery-life product, things like phone calls, cellular connection, sharp graphics and quality voice replies. Meanwhile the garmin products have fantastic readability outdoors on their low res, low power transflective LCDs.

They simply are on a different roadmap and each roadmap has its own constraints.