Because there's the constant struggle of making something so complicated that nobody wants to actually configure it and use it. So you have two options: sensible defaults and hiding things away, or just avoiding customization outright. And the second is always cheaper (for commercial products).

It's a failure of the UI and windowing frameworks to enable this level of programmable personalization. Designers chasing fads are just the opportunists that fill the gaps left by Apple's rigid design ethos, Microsoft's random gyrations, and Linux's ever-delayed Year of the Desktop.

As ridiculous as it first sounds, VR will likely provide the technology for the way forward.