I guess that's part of my question. A lot of car parts do come from the existing global parts supply chains, so building a "new" internal combustion vehicle wouldn't be that hard because almost nothing about it would be unique or proprietary and would be available from suppliers, right down to complete power trains.
With electric cars, though, there's a lot of engineering synthesis between the batteries, the drive train, even the braking (for regenerative braking) and in some ways, even the chassis considering the weight/safety issues relative to batteries.
While it's "just electric motors" and "just batteries", you're not building a golf cart, you're building a car where capacity/distance are major selling points and where innovation is ongoing, meaning that these systems aren't parts bin parts, many of them are highly proprietary engineered systems that can't easily just be bought off the shelf if they can be bought at all, especially when the buyer is Apple and the vendors of these products might not want to cede the market for electric cars to them.
With ICU cars, the incremental improvements in powertrains are miniscule, so nobody has a problem selling you their engines or transmissions and the rest of the car literally is parts bin parts from Delco or Bosch or the like.