Power and capability. Custom silicon means they can build the chip with all of the chip's surface being used for things their software needs (ie. extra surface for parallel computing and zero surface for unneeded features). They can also tune the chip with ops/watt in mind. Nvidia has touted their A.I. chips being x10 more powerful, but they also took twice the power to do it [1]. For an electric vehicle that lives on only the charge stored in its batteries, you want to use exactly as much power as you need to get the job done and no more.

https://news.ycombinator.com/item?id=19732974