Probably because Microsoft is a software company, and expects most folks (consumer-level) to get Windows 8 with a new PC or laptop or tablet, which will be Windows 8 ready (multi-touchpad, touch device, all-in-one, etc). Windows 8 is indeed designed more with touch in mind, but the expectation is that most (not all, but most) people who end up with it long-term will have touch devices (whether that be a touch screen or a touch pad designed for it, etc).
It's amazing the hate I hear about how Windows 8 doesn't work well in a traditional keyboard/mouse environment, but it does work well with touch. No sh*t!
- laptops have outsold desktops since ~2008, and tablets (iPads, mostly) have outsold most OEM's PC sales for the last 2 years. Microsoft is a software company, and the OEMs (and Microsoft too, I'd wager) want to get in on that market, including small mobile devices like phones up to the tablet and laptop devices.
I don't plan on running Windows 8 on my old PC either, but I do run it on a tablet and it really is better than Windows 7 on the device. I don't think PCs will go away, just like the mainframe didn't go away after the explosion of the PC, but the desktop PC has a bulls-eye on it, and it isn't even Microsoft's doing - consumers have voted with their wallets, and they want laptops and tablets far more than they want (or need) desktop PCs. What do you think Microsoft is going to do? Stick around and miss out on that? No, they'd be foolish not to make software for the devices consumers are buying (and their shareholders would crucify them for being as stubborn as some of us are too).
Whether we like it or not, desktop users are going to be the past, and we either get used to that and move to more modern touch devices in the future, or we switch away from Microsoft to something like Linux or BSD - not Apple, as they've been down the laptop/tablet/phone road for awhile now, and if they kill the Mac Pro, there'll be no turning back there either.