When the DDI was first proposed at that location I think the intent was to reuse the existing structure. Then somewhere along the way it was decided that the existing structure should be replaced, but they were far enough along with the design ideas for a DDI that they stuck with that plan.
At one point, the interchange between the QEW and Glendale was tied into another interchange between Glendale and the former Highway 8 immediately to the north of the existing QEW interchange. It's always been a weird interchange because of that.