I think it is formed that way to keep the length of the exhaust headers equal. Your rear exhaust distance would be shorter and more restrictive due tpo the sharp bend.
To make an under bike exhaust practical, you might need to shift the assembly forward a bit. make the front header pipe shorter and then cut the midpipe 10cms or so in front of where the balance pipe joins and re-attach the flanged join.
You could then form the pipe as per your picture, but you would still need a balance pipe before the two pipes join. Not easy, but with the proper skills, workable.
Would look stunning though. Hope you figure it out!