I mean, theoretically they could make money by selling additional services. Tesla sells "Premium Connectivity" for ~$10/mo, though it might be against the license agreements traditional manufacturers have with dealers to sell enhancements directly to the consumer.

IMO it's more about control over the user experience. You don't want your customers' UX to be dependent on the whims of Apple or Google, because now you're implicitly building a long-term dependency with a third party that may not be acting in your interests in the future. You're moving closer toward a future where the vehicle becomes commoditized, and now you have more trouble differentiating from competitors. And keep in mind: it's only very recently that the "Apple car" project was cancelled.

That said, traditional automakers are also famous (or infamous?) for sourcing tons of components (including infotainment systems) from the same parts manufacturers. But I guess at least that retains the ability to pivot and use it as a point of differentiation in the future.