Compared to options like that, building cars just looks like selling sugary water. Why not put 90% in the pot and really move things forward?
Job security? I bet the shareholders would revolt at the first sign of faltering on a $150 billion project. Sugary water might not be exciting, but it is nice and safe and won't get the C-level execs fired.
The biggest problem will be dealing with their influence over state legislatures. Tesla already upset dealerships; the idea of someone like Apple (with their capital on hand, experience, and influence at the federal level) following in Tesla's footsteps will be enough to terrify dealerships into some pretty heavy-handed action.
As for the supply chains, there are a lot of differences between the supply chain for consumer electronics manufacturing and automobile parts, but Apple's experience is still intensely relevant. I think it's likely that they'll have a lot of successes in dealing with it, despite those differences.
Supply chain -> Assembly (manufacturing) -> sales (Dealership)
Tesla's attacking laws that say that all new autos have to be sold by dealers. That's quite different, and only a few states have such laws.
If Apple ventures out of computing and into areas that need that kind of capital to be feasible, you start comparing lots of medium to low margin industries, the car industry being one of them.
When you look at Apple's brand, company focus and in-house experience, there's only a few industries where diving into could make it a relatively safe bet for the capital involved.
Cars make sense because they are complex technology, already very brand and UX focused in their usual sales processes, and there is a lot of money to be made in the optimization of the supply chain, all things which Apple is second to none.
There were surely other options and I imagine are still being discussed in Cupertino.
Large Scale Urban Development seems like a similar viable candidate, but the salescycles and product lifecycles are probably too much at odds with the rest of Apples business. But the prospect of an build to order Apple Campus must seem quite exciting to some people that are having a blast building the new spaceship.
Among other industries, i could also imagine the Hotel industry being on their marks for similar reasons.