Because this is how window managers work.
There is no facility, either at the ICCCM or EWMH level, to describe a window that:
- can be maximized
- always sits at the top of the stack
- cannot be resized
At most, you can ask the window manager to respect various hints, or you can keep track of the monitor geometry and available workspace size, and resize your window accordingly. Of course, all of these policies are not necessarily implemented by every X11 window manager, or they may be implemented differently; that’s how X11 works—“mechanism, not policy”.