iPhone 7 battery is dying fast why

Probably because it's getting old.

Batteries naturally lose their ability to take and hold a charge with time and use.

It's completely normal.

Check Settings>Battery>Battery Health. If it's 80% or below, the battery needs to be replaced.

ornett ~ If the battery of your iPhone is the original one, it quite likely needs replacement:

iPhone Battery & Power Repair

About genuine iPhone batteries

(Articles found by searching Apple's Knowledge Base.)

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