It is about expansion of the ecosystem that people choose when they decide to go all in on some products.
Also, it could be argued that Apple have perfected the art of selling and extracting maximum value out of mobile phones, so they need to expand.
Also, a car is the second largest purchase most people will make after a house, so their emotional attachment to the vehicle is significant, which likely means people will spend money on "customisations" (think accessories, apps etc).
Also, vehicle manufacturers are trying to go the route of software developers by charging subscription costs for features on vehicles. I understand you can no longer buy "full self driving capabilities" with Tesla, you need to licence it, I've heard of other manufacturers off options as a subscription rather than an upfront cost.
Also, Apple will not only own the ecosystem (iOS and account / identity / payments), and the app store market place (30% fees / commissions on all sales), and the licencing for all accessories (Christ knows what they charge), they'll also have the entertainment ecosystem (the App Store) and multiple years to refine how they maximise value from each of these channels.
The car could almost be considered a secondary consideration - or even a loss leader in the equation (not that Apple would have had any issues in maximising vehicle profits given the combined experience of their team and partner network) should they have been able to crack the first generation product.
Anyways, they couldn't crack the code, so that's what they are walking away from, not just a $100k+ car sale.
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