I want to ask why iPhone X has Face ID where iPhone 8 or earlier iPhones do not? Why can't Face ID feature be imported to iPhone 8, 7 etc. via updates?
It's hardware. Go to Apple's iPhone X page, specifically the page on Face ID.
A tiny space houses some of the most sophisticated technology we’ve ever developed, including the cameras and sensors that enable Face ID.
The TrueDepth camera system is composed of multiple innovative technologies. Working together in real time, they use a detailed depth map of your face to recognize you in an instant.
More than 30,000 invisible dots are projected onto your face to build your unique facial map.
An infrared camera reads the dot pattern, captures an infrared image, then sends the data to the Secure Enclave in the A11 Bionic chip to confirm a match.
Invisible infrared light helps identify your face even when it’s dark.
It's not simple a software update. To add Face ID to other iOS devices would require a hardware upgrade.