Because Microsoft sees tablets as a type of PC, not an overgrown phone like Apple and Google. They want a unified experience.

Also? With the "metro" interface as part of the core OS, they're basically forcing manufacturers to start including touchscreens in laptops and to start selling touchscreen monitors. Which isn't exactly bad for manufacturers since it means people will want to upgrade.

Those are my opinions, at least. Personally, I agree that without a touchscreen, the new interface is annoying.