Why is iPhone 14 using iOS 11

Make sure that you are buying from an authorized Apple reseller by verifying that the name of the company appears in this list of authorized Apple dealers:

Find Locations

Otherwise, the phone is not likely "new" and it may or may not be covered under the AppleCare warranty.

Yes, it was released with iOS 16 and would currently be running iOS 17.x.x It’s either a counterfeit, a misprint or something worse. But a genuine iPhone 14 Plus could not load or run with iOS 11

From what site or business are you trying to purchase this iPhone? Most likely it's just a typographical error, but without knowing exactly what you're seeing and where, we can only guess.