That it's hard. Lithium ion batteries at scale is a difficult manufacturing process. All the R&D is going into battery. Essentially the electric engine is easy, but the battery is hard. It's exactly the other way around than in classic cars. Imagine if all major car companies would be buying engines from a few Asian tech giants.
There is quite some debate on how much of the profits will go to the battery manufacturers eventually, but I've seen 30-70% from serious analysts. I think nobody really believes the top line number, but even the more reasonable 30% would be massive.