... Because the attic isn't part of the living space. As The Insulator said, the 'envelope' around the living space should be insulated. That would EXCLUDE the rafters.
The downside to insulating the rafters is you don't ventilate the roof. IE., in the summer, it will get incredibly HOT, and in the winter it will STILL get hot, then, you'll have an icedamning problem. If you're wondering how the attic space is finised in an attic that's part of the LIVING space, soffit chutes are installed on the underside of the roof sheathing. They create a contiguous 'chute' from the soffit to the ridge vent. This chute allows air to flow into the soffit, along the underside of the roof, and out the ridge vent, thus cooling the roof. Insulation is installed over the chutes but having the chutes and vents at BOTH ends are key.
In short, my $.02 says you won't gain anything from insulating the roof even if it's done w/chutes. I say it will harm the roof. My best to ya and hope this helps.
PS: Sorry for the long delay in responding. My 4 year old had surgery yesterday and I was out w/dealing with that. She's just fine, thanks.