Are you serious or is this just inane bitterness?
(a) We're talking about an SUV here, not a sports car. The design tradeoffs for the powertrain are going to be very different.
(b) Even if the Tesla Roadster really were that close to being the tzero, which is something I'm not convinced of, and the Model S were that close to the Roadster, Tesla's still the company with the integration and manufacturing experience.
(c) Tesla owns a manufacturing plant whose purpose is to produce powertrains for the Model S.
The electronics and control are the easy part. People who haven't thought about it don't really appreciate how much work goes into setting up a production run for machined metal parts: beyond the basic mechanical what-goes-where, there are tradeoffs in material choices, choices on which parts to source stock and which to manufacture custom, arranging manufacturing capacity and tooling up the plants where the production will be done, and QCing the finished product.