Ask HN: Why does Apple use an architectural license from ARM at all?

Many articles explain why other companies buy ARM IP in order to use reference ARM CPU designs as-is, or to make incremental modifications or extensions to those; and yet many articles claim Apple is nontrivially re-implementing the design to achieve the performances and power efficiencies of the M series chips.

Other articles also explain the advantages of compatibility with a preexisting ecosystem of ARM-compatible devices; but, that does not strike me as a primary compelling reason why a major company would care, other than many of Apple's own mobile devices were already ARM architectures? Let's say it makes sense for a laptop and a phone to share a unified processor architecture so they can run the same code--then Apple could have even made their own "Apple Instruction Set" instead?

Is there some kind of technical advantage to the ARM architecture over say, licensing the RISC or the (supposedly aging) x86 architectures? Or is it that there are more ARM architecture engineers out there? Are Instruction Set Architectures so difficult to design, that a special company needs to own and maintain the IP for it?

After reading a bunch of articles I did not feel I actually understood this at all. Many kind of touched on it circumstantially, but I felt they did not directly explain the advantage or rationale for doing so the way Apple did (I thought of the PowerPC but I assume in that case, Apple/IBM/Motorola worked together to specify an architecture). I'd appreciate any insight or pointers to actually good articles on this.