because the blue print is already there and they don't know what to replace it with. There are also many things you can plug in there besides an optical bay. Eg.. Card readers, Audio ports, Fan controllers, Watercooling systems, etc...
It's legacy, it's easy to just put it in, it helps with chassis rigidity, it may upset some customers if they replace it with something else. Until there's a clear standard to replace it, case makers won't replace it.