Presumably because ARM’s ISA is more advanced and costs Apple basically nothing (I’ve heard it might be as low as six or seven figures for their architectural license).
*Microsoft, Apple, Google, Amazon, FB
[Edit 1]: A quick search throws up that Google, IBM, SONY etc are already members of RISC-V foundation.
Also, https://riscv.org/2018/08/eet-asia-article-microsoft-and-goo...
Why would a nation choose to go to the moon, or build something otherworldy?