How can I show the menu bar on all displays in macOS?

No one here has the inside scoop on who made that decision.

But as a developer you can sign on http://developer.apple.com/ and make a bug report through their feedback system. You will be subject to their non-disclosure agreement, and not able to report those bugs on this community here.

One thing you might want to consider is develop for X11. That does have the option to build menu widget windows based on the BSD system. http://www.sourceforge.net is an excellent repository of Mac compatible open source software using X11.

Historically Apple's menubar is a holy grail for the Mac platform, where unlike Windows and some Unix platforms there is just one menubar for the active foreground application, and the menubar always has altered to represent that menubar. While I can certainly see where the flexibility of having more than one application with its own menubar can be useful, it can also be counterproductive for people who enjoy the Mac interface intuitive design. The KISS principle weighs heavily into that design. In fact until 15 years ago, almost no applications hid the menubar, except some games. This made it difficult to quit those programs if they misbehaved.