Update iOS 17 on my iPhone 8?

Because the iPhone 8 is not compatible with iOS 17. That is not going to change.

See the list of devices that are compatible with iOS 17 near the bottom of iOS 17 - Apple

As noted you can’t get iOS 17 on an iPhone 8.

An iPhone 8 or iPhone X isn't capable of 17 or above; they require an A12 or better CPU.

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iOS 17 will never be available for the iPhone 8.....because the iPhone 8 hardware cannot meet the requirements for iOS 17.

How to I update my iPhone 8 on iOS 17

You don't. The answer has already been given three times.

This is not possible.