If you are an automotive company you need a self driving plan (not always a division) as risk control. If your competitor comes out with a self driving car that is statically safer than humans you might find your manual driven cars banned from the road. Having some IP in self driving cars is thus a good way to ensure that should that happen you can get good license terms for their tech (play nice with us, or we will sue your for patent violation). Note that good license terms can be millions of dollars.

A plan means either you have a division, or you have an agreement that you can use someone else's if it works out. Either works.

As car makers discover self driving cars are hard they worry less about the risk and put less into development.