Why my iphone werranty expires

Because it is a limited warranty. Most of the products Apple makes have 1-year limited warranty. Here you can find more info Apple One (1) Year Limited Warranty

You can extend that with Apple Care plans Buy an AppleCare plan - Apple Support. if you don't do it, you'll then have to pay for services you need to your devices after that 1 year, except Service Programs opened by Apple itself, see Apple Service Programs - Apple Support

Because limited warranties have an expiration date… The warranty is for one year. One year from the original date of purchase, the warranty expires. That’s how warranties work…