Well, for one thing, output sharpening is resolution- and device-dependent. When it gets right down to brass tacks, you'd sharpen differently for different papers on the same printer and at the same resolution if you're at all interested in making the best possible print — a printer/paper profile may compensate for colour and (to a somewhat lesser extent) density range, but it won't compensate for dot gain and its effect on apparent detail rendering.

But to go a bit further: capture sharpening is essentially compensation for flaws in the process, from minor lens flaws to demosaicking and optical low pass filtration, to in-camera and post-processing noise reduction. Regardless of your final rendering intention (how much you wish to emphasize of de-emphasize detail and local contrast), you'll want the best possible baseline image to work with. The reason for doing this up-front is that your rendering intentions may change, and you want to establish a baseline as few times as possible in the process.

Neither output sharpening nor capture sharpening require much in the way of judgement; they're essentially "mechanical" processes applied to a two-dimensional image, one to get a good image to begin working with, and the other to render your final intentions to your chosen output media (on-screen at various resolutions as well as printed at various resolutions on various papers using various processes).

Creative sharpening is a separate process because it's not "mechanical" (unless you're the sort of "purist" who believes that you should take what the camera gives you and be thankful, in which case failing to shoot JPEG rather than raw is already showing a degree of hypocrisy). This is the part in the process where you choose your film (or combination of films), so to speak. And yes, depending on the software and workflow you're using, this can become irretrievably mixed in with capture sharpening (but, one would hope, not with output sharpening, unless you only ever render to one device/medium at one resolution). When there is a choice, you'd want to keep it a separate step so that you don't need to redo the capture sharpening when you change your mind. But creative sharpening is more about establishing contrast than finding and refining edges; it's working to give the illusion of a third dimension (or, in the case of things like skin smoothing, to selectively suppress the intrusion of the third dimension into the two-dimensional representation).