This is basically the crux of the matter. Traditional automakers are a complex supply chain that standardises and goes to extra-ordinary lengths to reduce costs. Tesla build a tonne of their own stuff and aren't as price sensitive. The question is "Why don't we just redesign all this stuff" and the answer is "We're Ford, we make tiny margins and we can't afford to redesign our entire car every year and even if we could we get half our components from Bosch anyway". Not to mention the difficulty in convincing FuSa people your arbitrary ethernet network is safe.
In cases where they're using a proprietary 48v part I wonder if Tesla would consider becoming a parts supplier to other manufacturers?