OK! Let's invent a reason since basically, everyone agrees it looks a bit silly.
While opening the door on the freshly finished prototype 67 #1 Mustang with only days away from full production start of the 67 Model year, an engineer scratched one of his/her freshly manicured nails on the adjacent window crank handle and took this back to the review committee as a defective design flaw that "must" be addressed. The committee rejected the notion of making yet another change to the early 67 models because there were already countless flaws of greater significance. They instead decided to incorporate it into the vast amount of changes to come out in the 68 model year. The decision was handed down to point them downwards thus ending a two-day debate.
Well, it makes good sense to me! But I am no engineer!
Richard