But OS X seems like it would be hard to control with a touch screen. ...
How you figure that?
... Maybe they'd have to modify the OS.
MacOS X is object oriented. Almost everything that you need for a table computer is already part of MacOS X. Don't forget that the iPhone/iPhone 3G and iPod touch run OS X. A larger format tablet may need to have some existing features fleshed-out, but it would not require major additions or rewrites.
The fastest way for human beings to enter data into computers is to type it in.
Inkwell will work for certain applications, but not for general-purpose computing. Sophisticated
Multi-Touch gestures greatly enhance the computing experience, but they are not substitutes for traditional data-entry methods.
Apple has not introduced a Mac tablet because there is virtually no evidence that a market for such a device exists.