Is there a reason why my iPhone's battery wears out fast and sometimes dies not at zero percent?

You may need a new battery. Theres a lot of battery replacement kits if you wan to fx it yourself.

the battery is degrading, this could be the result of old age, or leaving it on the charger too long. Either way, be prepared to take your iphone to a repair shop to replace the battery.

The battery degraded to a certain point. You could check the health on iPhones running iOS 11 or greater. If its 80% or lower should look into getting it replaced.

Yes...

It should be replaced.

Try this quick solution just in case one the app settings are stuck or misbehaving-Go into Settings then to General and tap Reset then choose Reset All Settings then enter passcode if have one then.Confirm the Reset and Confirm once again. If still draining fast it could be brightness up to much or to much streaming or plain out bad battery

As batteries age, they loose their capacity to hold a charge. So the battery will die faster and may take longer to charge. There really isn't anything to do to "fix" this as the battery has to be replaced.

If you constantly use apps that use GPS, high brightness, games, etc, it will drain you battery faster.

That means you have a bad battery. All phone batteries will start to deteriorate after a couple of years. They replace iPhone batteries at any Apple store and every local cell phone repair shop will also replace iPhone batteries for you. It can cost between $30 and $60.

If you use power hungry apps and processes it uses up the battery fast.