Horace Dediu, writing for Asymco, starts by defining the term Significant Contribution, as laid out by Tim Cook:

We believe that we need to own and control the primary technologies behind the products we make, and participate only in markets where we can make a significant contribution.

Dediu works through some numbers, but lands here:

An electric automotive system requires new infrastructure, new user behavior, and, in the case of Tesla, new distribution network and new post-sales support network. Offsetting this partially is a government subsidy.

In contrast, the smartphone communications and computing system leveraged existing infrastructure, consistent behavior, at least initially, and the same distribution and support network. In addition, smartphones benefited from significant hardware and software and services ecosystems which encouraged third parties to add value to the system.

An interesting read. I’d argue that Apple is painstakingly cautious about entering new markets. If they decide to build a car, my bet is that they’ve already worked through the risk and cash flow numbers, done the market research to assure that the math works, that the risk is small enough for them to go forward.