I was just in a discussion on this very topic. It's the build vs buy equation applied to silicon. Early in the tech boom the entire silicon stack was proprietary and required a lot of time and investment to train up people who could design the circuitry, we got our first "ASICS" which was basically a bunch of circuitry on a die and you then added your own metal layer so it was like having a bunch of components glued to a board and you could "customize" it by putting wires between the parts. Then we had fabs that needed more wafer starts so they started doing other peoples designs which required they standardize their cells and provide integration services (you brought a design and they mapped it to their standard cells and process). And as the density kept going up they kept having loots of free space they needed to fill up. The 'fabless' chip companies continued to invest in making new parts until the pipeline was pretty smooth. And at that point the level of training you needed a the origin to get it into silicon dropped to nearly zero, you just needed the designs. And into that space people who were neither 'chip' companies, nor were they 'fabless' OEMs, realized they could get their integration needs met by asking a company to make them a chip that did exactly what they wanted.
One the business side, the economics are fabulous, your competitors can't "clone" your product if they don't have your special sauce components. So in many ways it becomes a strategic advantage to maintaining your market position.
But all of that because the all up cost to go from specification to parts meeting the specification dropped into the range where you could build special parts and still price at the market for your finished product.
A really interesting illustration is to look at disk drive controller boards from the Shugart Associates ST-506 (5MB) drive, to Seagate's current offerings.
It is illustrative because disk drives are a product that has been ruthlessly economized because of low margins. The ST-506 is all TTL logic and standard analog parts, and yet current products have semiconductor parts that are made exactly to Seagate's design specs and aren't sold to anyone else.
So to answer your question; apparently the economics work out. The costs associated with designing, testing, and packaging your own silicon appears to be cost effective even on products with exceptionally tight margins, it is likely a clear winner on a product that enjoys the margins that electric vehicles offer.