Jim, the main reason I did it this way is because I have a mill and I wanted to play with it
. Another reason is because it's easier to 3D model one part instead of having to draw and assemble multiple parts on the computer. With a built-up design, it could be a little harder to keep the inside and outside pieces aligned with each other if a bolt hole is just a little off. Although I suppose you could bolt the pieces together and then do all the machine work on them after. I would design a little fudge factor into the overall dimensions of the rough parts so they could be squared up with the mill after they were assembled if need be.
Actually, milling out the center was the easiest part. The outsides took the longest.