In fact, Ballmer just announced that they are actively pursuing Metro-izing Office. http://www.computerworld.com/s/article/9220091/Microsoft_CEO_hints_at_Metro_ization_of_Office
I'm sure they'll do it, but it'll likely be a cut-down version of Office similar to previous cut-down versions of Office and there will still be regular desktop Office.
They have hard drives and big, high-res screens, and on top of all that, I can use the digitizer to draw and sketch and take notes. A tablet is an underpowered, overpriced screen backed with annoying specs, a generally weak digitizer, and a painful OS. No keyboard. No desktop-OS applications (except, I guess, with Windows .
My tablet has a 64GB SSD, 1366x768 display, 2GB RAM and a 1.66 GHz CPU. It may not have a physical keyboard but I can plug one in if need be (and that would be far better than any physical keyboard that you would get on a laptop). I've never been a fan of laptops - they're too uncomfortable for serious use, and too bulky for practical portability. Having a desktop and a tablet puts me comfortably at both ends of the spectrum.