

Why? Because having a keyboard flat with the surface of your desk is better for your hands. Having the keyboard tilt up towards you is a bit more stressful to the wrists. Go on, try it and see.
With a flat keyboard your wrists are more or less straight, but with a tilted one your wrists are bent upwards. Over time this can be uncomfortable. When I pointed this out to Steve he said, yeah, true, but the market wants curved keyboards - it looks more sophisticated. And besides it's cheaper to make - fewer parts to assemble, you see.
It's like a V6 engine vs. a straight six: the straight six is actually better (see BMW) but the reason why many car companies make V6s is that people think they're more advanced or some nonsense. You can't undo a V6 but at least with our older keyboards you could prop them up to make them flat again.
Just an example of the small compromises that we have to make.