This is a user-to-user technical forum. Contributors here have absolutely no insight into Apple’s design decisions or its product development or release roadmap. As such, we cannot comment upon “why”.
Your iPad when connected to an external monitor will “mirror” its screen to the monitor - and while doing so will preserve the screen aspect-ratio of the iPad. As the external monitor will have a different aspect-ratio, you will very likely see black infill bands. Some external monitors have the ability to stretch the display image to fill the screen, however, this comes with the penalty of introducing image distortion through stretching the image horizontally or vertically.
When playing video material from some Apps, instead of providing a “mirrored” output, iPad will instead output a standard widescreen image - for which most monitors will attempt to show as “full screen”. That said, many widescreen images don’t fully match the monitor’s aspect ratio - and will inevitably be show with infill bars at top/bottom, or will be expanded and cropped to fill the screen.
Returning to your comments concerning an extended desktop, this concept has issues in context of a touch-screen environment. As user-input requires targeted touch interaction with the screen, you’ll not be able “touch” or interact with elements that are visible on the external monitor. As such, an extended desktop per-se isn’t viable.
Apple do invite submission of constructive comments and feature requests via its Product Feedback portal:
Feedback - iPad - Apple