They have a well established ARM based ecosystem (10+ years of development of their own custom arm-based designs), and RISC-V is not real world proven.
ARM has a massive number of developers and has a 35+ year history for the kinks to be worked out. It was available on end user computers in 1987.
Apple going RISC-V would be absolutely insane. The licensing fee(s)? Not a big deal (I mean, ARM based processors are used in tiny cheap devices, so it isn’t much), and Apple’s customers aren’t super cost sensitive.
They’d be throwing away decades of base-level ARM refinement, and at least 1 decade of their own development. For what? What’s the net win for Apple here? An open platform anyone else can more easily clone? Why would they even want that?
If RISC-V was around 15 years ago, maybe they would have been on board. But that ship sailed in 2005 or so, and Apple committed to what was available when they needed a new arch. Right now the reasons to switch just aren’t there.
Ditto for Google with their hardware too btw. RISC-V is a science project at the moment, and until somebody builds something significant with it (or at the very least, until its full instruction set is sorted out and standardised - privilege levels weren’t even sorted out until mid 2019) and there’s a significant developer community with significant OS support for it, it’s a non-starter really for someone building real multi-tasking OS products at scale like Apple or Google are, when ARM is available and well proven.