If other companies could legally figure out how to license and build x86 chips, nVidia would already be there stamping them out. It's a legal minefield that's simply not worth exploring, even for Apple.
...to compete with Intel? Why would Apple want to and nVidia not want to? Apple sells computers, nVidia sells chips.
Apple loves vertical integration and all, but selling chips is the reason nVidia exists. Apple could put out a clothing line, switch to building luxury watches or cars, or commit entirely into doing "fashionable audio" (Beats, Apple Music) to compete with Bose and still exist past computers - it's a lifestyle/fashion company masquerading as a tech company. Apple making chips is means to an end - to fill a hole in the market that Qualcomm and Samsung weren't able to. If it were anything more than that, Apple would be taking customer orders for A11s.
Meanwhile, nVidia's Project Denver and the whole Tegra stuff only came about after their plans to attack Intel head-on failed miserably. They hired up a bunch of Transmeta employees in the hope they could implement a binary translating CPU that would "cheat" them into being able to implement an x86 processor without the licensing stupidity. Intel's lawyers proved them wrong.
