My watch doesn’t last all day. Very frustrating. Why does this happen

Series 6 and Series 7 battery life are roughly similar (~18 hours), and here is Series 7:

Apple Watch - Battery - Apple

That link describes expected battery usage.

Things that can consume more power include use of cellular radios, bright display settings, and Watch operations beyond parasitic range with the paired iPhone.

Apple Watch Battery and Performance - Apple Support

If you’re not getting the duration Apple documents, make an appointment with Apple Support for a look at the Watch and its battery.

You’ll probably also receive a suggestion to un-pair and re-pair, as that will reset the Watch, and that reset can sometimes clear odd problems.

Batteries - Maximizing Performance

You should consider making a "User Tip" from your "Battery Saving Tips."