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They probably do not have the rest of the interior components (dash, door panels, etc) available in white (yet). The other colors already exist and the interior parts would already be in place.
The rest of the interior is available in white, it's just not the ventilated/perforated seats in white. Currently only the solid (non-perforated) white is available
I was wondering this same thing! I have a hypothesis...
Tesla has been having some issues with the perforated ultra white seats on the Model X, and I bet they are holding off on offering the perforated ultra white synthetic leather in the S until they can work out some of the strength and durability issues.
The above thread is just from this month, and an attached pic shows a small tear around one of the perforations. My guess is that the SCs have had to repair or replace a lot of the perforated ultra white seats in the Model X but the non perforated ones are proving to be more robust (since the material isn't real leather, the perforations are perhaps a point of structural weakness??).
Making the seats in house is probably a source of significant cost savings for Tesla, but not if they have to continuously replace a whole bunch of the seats under warranty!
My theory is that the demand for the white ventilated seats in the X is eating up much of the supply, so they can't afford to offer them quite yet into the S. I assume they will eventually.