Most new Wacom Cintiq tablets have touch, so when you attach them to a Mac, you basically get a touch Mac. There even is a 27" touch Cintiq. I use my Cintiq Companion 2 attached to my iMac. I can use a touchscreen in OS X without problems, it's not hard to press things, use a finger instead of a mouse, and it's even in a comfortable position as the Cintiq is mostly horizontal...
...and yet, I never use touch. Simply put, it's not needed. The mouse, keyboard and a stylus may not seem that fancy, they are the best ways to control your Mac. The reason an iPad or an iPhone use touch and not something like a mouse is not because touch is better - it's because it would be hard to use a mouse with devices you hold in your hands (imagine having to pull out a mouse in the middle of the street to use your iPhone).
What I'm saying is - every device is different, every usage case is different, and at this time, we just don't need a touch-based iMac. I mean, it's especially silly for an iMac - I don't know how the OP got the idea, but it wouldn't be practical at all. If they made a hand-held Mac (like a tablet), then perhaps touch would make sense. But on an iMac?! Seriously?
BTW - I have actually used touch on a gigantic iMac-like computer: the HP Sprout. Using touch on it was very unpractical.