Because they'd be REALLY ugly on a Spyder. A Spyder cockpit is quite narrow compared to a Speedster. But they use the same windshield.
The door tops you seem intent on Spyders using would be about 2" wide on top of the door and extend outward: UGLY.
A Spyder is supposed to be a spartan race car, not a Speedster or a Roadster D. The few creature comforts mine has are hidden from view: heat, defrost, heated seats, and Bluetooth amp/speakers. If I ever get around to finishing the top, it will follow the same ethos: unobtrusive and only noticeable when installed/in use.
The side curtains/windows must sit a couple inches from the inside edge of the door. I still think a couple ferrules will look just fine and not be very noticeable.