Tali Roth, the then product manager working on the core Windows user experience, including the Start menu, taskbar, and notifications, took up the question and talked about how building the taskbar from scratch meant that they had to cherry-pick things to put into the feature list first, and the ability to move the taskbar didn’t make the cut, for several reasons that Microsoft values.
WHY WOULD YOU DO THAT?!
If you have working code, why would you rewrite it from scratch? Refactor, sure. Overhaul, maybe. But why rewrite the whole thing?! You're gaining nothing but unnecessary bugs.
I know all the joke answers. To justify a product manager's salary, because Microsoft gonna Microsoft, whatever. I want to know the real reason. Why would you ever rewrite working code from scratch if you don't have to?