Because, at that C-channel butt joint, one piece is curved
C-channel and the other is straight
In general manufacturers use stock metal extrusions, and do not fabricate "curved and
straight in one piece" parts for things like this, unless there is a clear and overwhelming structural reason to do so IMO.
In this case the C-channel is not structural, and would not prevent rear-end separation, so why go to the expense to have one long "curved and
straight" C-channel? [assuming I am seeing this correctly]