So, that's why. Up until now Windows logos have been designed to be graphics bling, not to look good. Never trust a designer that uses "because we can" as a design rationale, or justifies designs with "why not?". Its all good and well to explore the boundaries of what can be done, but when that is the only thing you're doing you're simply not designing anymore.
Windows team blog, Redesigning the Windows Logo:
In some ways you can trace the evolution of the Windows logo in parallel with the advancements of the technology used to create logos. From the simple two color version in Windows 1.0 to the intricate and detailed renderings in Windows Vista and Windows 7, each change makes sense in the context in which it was created. As computing capabilities increased, so did the use of that horse power to render more colors, better fonts, and more detailed and life-like 3D visual effects like depth, shadows, and materiality. We have evolved from a world of rudimentary low resolution graphics to today’s rich high-resolution systems. And what started as a simple “window” to compliment the product name became a flying or waving flag.
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The Windows Vista release marked the beginning of the AERO design aesthetic in Windows with a key component of the interface being the “AERO glass” effect. Replacing the green Start button was the round glass-like button with a now flattened version of the “flag” from Windows XP. Internally, this icon became known as the “pearl”. You can see the intricate lighting effects of the faux glass. In many ways signaling just how powerful of a rendering engine the PC had become.
And with Vista they discovered the Aqua, no sorry, the AERO glass effect.