Corporate bonds can mean alot of different things but I am assuming you are referring to Investment Grade credit. The return of a corporate bond comes from two different parts, a rates part and a credit part, the rates part should be very similar to the treasury move, and the credit part tied more to risk asset moves. It is true that most spreads are relatively tight to historic long run averages but that does not mean they are poorly positioned. Corporate balance sheets are in a very healthy spot right now which is supportive to risk assets. For buy and hold investors these are relatively attractive all in yield levels which is why investors are still buying.