Why on Earth is this a requirement? When you're navigating through Finder using keyboard, it's very inconvenient to use two keypresses to perform a very basic operation. Using Enter to open a file is how every file manager on every operating system works except Finder. Why would Enter key be hardcoded to a file rename operation instead?
It is a typical Apple behaviour of doing things differently from the rest of the world just for the sake of it, even when it's detrimental to the user experience.
Actually I just checked and it's not, technically you can create key equivalents without modifiers as well [1]. For Finder this doesn't work though, because enter seems to be specifically handled before menu-level key equivalent processing. (Note that it's not guaranteed to work on other apps either, based on [2] seems key equivalents are only dispatched if modifier keys exists. But that might be out of date since it worked for the people in the SE post.)
Option+Enter is the next closest thing.
I agree that their implementation here is not good. In fact there's already a "Rename" menu item, which isn't actually wired to the enter hotkey (this is very un mac like because it means there is no easy way to discover it). The "rename" menu item is actually a fairly recent addition to mac (I think maybe 10.11) while Finder itself is ancient (it was one of the last few apps to be migrated to Cocoa and even today still has lots of legacy warts), and possibly no one bothered cleaning things up.
[1] https://apple.stackexchange.com/questions/132984/keyboard-sh...
[2] https://developer.apple.com/library/archive/documentation/Co...
