cant update my iphone 7 plus

As to "why", I'm guessing it's because they wrote iOS 16 and up for the "Bionic" family of chips, and the iPhone 7 has an A10 "Fusion" chip. See https://en.wikipedia.org/wiki/List_of_iPhone_models#Unsupported_(64-bit_CPU,_2013_to_2017_models)

[Edited by Moderator]

You can update an iPhone 7 Plus to the latest release of iOS 15. You cannot update an iPhone 7 Plus to iOS 16 or higher.