Why on earth would they want to do that? Apple sells hardware, Microsoft doesn’t. Apple has an interest in people only running OS X on Apple hardware. Microsoft has an interest in selling as many copies of Windows 7 regardless of what hardware they run on.
"Microsoft has an interest in selling as many copies of Windows 7 regardless of what hardware they run on."
Case in point:
That's a Macbook Pro running Windows 7 behind Microsoft's CEO.
And why not? MS gets its revenue from the OS sale, not the PC sale. Apple is a hardware manufacturer, Microsoft mostly isn't.
Incidentally, I use Win 7 on a Mac all the time. I can boot into it natively, but if I run it in a VMWare virtual machine I can switch back and forth between the two OSes. Works great, but there's just no comparison.
Microsoft has an interest in selling as many copies of Windows 7 regardless of what hardware they run on. But one of the selling points Apple advertises is that Macs can run Windows. Microsoft gets to take down an Apple selling point, and only has to forego a tiny, tiny percentage of its sales of Windows.