You mean all the other OS's reverse engineering Cocoa? That exists, more or less, in the form of GNUstep. There are good reasons why GNUstep isn't heavily used: it doesn't feel native anywhere but macOS, macOS apps tend to use a lot of OS APIs (e.g. Metal) that are outside Cocoa so just implementing Cocoa doesn't buy you much, Objective-C isn't that productive of a language by modern standards, etc.
More realistic, honestly, would be for everyone to implement Win32 as a lowest common denominator, which is kind of the world we live in already thanks to Wine.