I don't understand why my battery health dropping so fast

Batteries degrade over time with usage, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

Why lithium batteries degrade over time.

https://airqualitynews.com/2021/01/12/why-do-lithium-ion-batteries-degrade-over-time/#:~:text=These%20are%20often%20lost%20through,be%20achieved%20by%20the%20battery.

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple Batteries - Maximizing Performance - Apple