why does my battery health drop so fast?

Battery Health will never increase. It will always go only in one direction......down.

The only way to increase the Battery Health would be to to replace the battery. Then, the Battery Health will be 100% and start to go down an average of about 1% a month.

When the Battery Health is down to 80%, it will be time to replace the battery again.

As lithium-ion batteries chemically age, the amount of charge they can hold diminishes, resulting in shorter amounts of time before a device needs to be recharged. This can be referred to as the battery’s maximum capacity – the measure of battery capacity relative to when it was new. In addition, a battery's ability to deliver maximum instantaneous performance, or "peak power", might decrease.

Source: iPhone battery and performance - Apple Support (IN)