No car maker dares not be in the race. There is the very real risk that somebody in the race will release an autonomous car, and in just a few months(worst case, but it is believable) governments will see those car is significantly safer and mandate it on all cars. If that happens any manufacture not close to finished won't exist anymore.

Note that being in the race just means they have a plan in place. Car makers partner all the time - it is common to see the same car from several different makes with only the logo changed, or an optional engine from someone else.

I expect that in the end there will be about 5 different self driving systems on the road that are shared between different cars. Designing a system is complex and expensive on the one hand; and not a differentiator that customers will pay for on the other hand: so it won't be worth the cost to continue to develop a system if you have it when you can buy from someone else. (though each might customize the UI as the UI is a feature they can sell)