Why my phone don’t wanna update

Sorry, but an iPhone 8 can only run up to iOS 16.

See:

iPhone models compatible with iOS 17 – Apple Support (AU)

iPhone 8 doesn’t supports ios 17

Because an iPhone 8 can't run a higher operating system than 16.

My iPhone doesn’t want to update to iOS 17

That would be because your iPhone 8 Plus understands that it cannot update to iOS 17.