Because programs can not register an icon to minimize the apps to the menubar app while providing core functionality and info via a context menu and tooltips, at least AFAIK.

Spotify can do that, so apparently it's an option even though not very common.

Any app can, provided they build a menu item. Most apps don't, and those that do don't usually connect the presentation of the menu to an action in the app, it just runs alongside the app (e.g. Kaleidoscope). But you could do that if you wanted.

Interesting to know, thank you.