I believe it was in the Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future (audio-)book that I heard that at least for SpaceX, but I think also for

I believe it was in the Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future (audio-)book that I heard that at least for SpaceX, but I think also for Tesla, building the components in-house came out much cheaper compared to what their competitors would have to pay for the same components. They were often more advanced, too, because the component could use whatever was most modern technology and was highly-optimized for SpaceX/Tesla's use case. In contrast, the components for other carmakers have to be much more uniform and satisfy everyone, as you said. When it's a new type of product, it usually pays to become a system integrator in order to squeeze the maximum performance out of every component in the system and benefit from the synergy. The other carmakers haven't done that yet with EVs, and they were in an even worse shape a few years ago when they were still only repackaging their ICE cars into EVs. Whenever they would do that, not only were they able to use only small battery packs inside those cars, because they weren't designed to be EVs from the ground-up, but they would also be significantly less efficient per kWh of battery pack. By the way, that book is fantastic. I highly recommend it.