Is it because legal or licensing issues? is the technology not available? If this is considered being implemented can someone suggest a date?
UE4 uses OSX SDK / Cocoa API, and those are not available for windows, so that’s one reason why it’s not possible to build osx version of the game in windows.
The reason they’re separate is that xcode is need in the process for packaging the OSX version, and VS2013 for Windows Packaging. If you have the source version it may be possible to set up to packaging for the other OS, but you would be on your own to set that up. The functionality to package between OSX and Windows is on the list but when/if has not been specified.