At least on D1- there are really two camlock components. The spindle has a number of radial holes that intersect the axial holes that accept the camloc pins. This is all in a ring that is usually one solid piece with the spindle taper. (The other piece is essentially part of the chuck, and had the camloc pins threaded into it.)
I think you are referring to the spindle side of things. Since the camlock face of the spindle is a precision part of the whole assembly, and needs to have a very accurate relationship to the external taper of the spindle, they are made as one piece. The camloc adaptor that is part of the chuck is supposed to BOTH fit flush to the spindle face AND seat on the external spindle taper, to a high degree of contact. I have never even figured out how the heck they do that to any level of precision, but it obviously is a very critical machining task!
The spindles are frequently cast and then machined to size.
I've never seen the separate piece design, but my guess is it was assembled first, before the spindle was machined, and is designed to never be separated.