The benefit of having control is that they can adapt them to their priorities. Similar Apple designing its own chips when there were already viable producers in the market.

They won’t need to rely on others prioritizing their priorities, like low volume, high cost early investments in batteries designed for a market (humanoid robots) that doesn’t exist.

If they then scale them up, they also have the benefit that there is no 3p supplier who can turn around and sell those to a competitor.