Why?
They already have 90% plus of their products using ARM, already have an ARM based CPU development team and already have heavily customsied-to-their-requirements ARM CPUs, a well tested ARM compiler toolchain, libraries, etc.
RISC-V is cool, but it is, as yet unproven and would result in Apple starting from scratch with any custom stuff they’ve been working on and be totally incompatible with the rest of their product stack. They’d be throwing the past 10 years of in-house CPU/toolchain development out the window.
Using RISC-V would be an entirely ideological choice (“wow, it’s cool!”), not a practical one, and Apple aren’t making these decisions for ideology reasons really. It’s entirely business costs, performance and risk mitigation.
Right now RISC-V is not battle tested, the resources, documentation and experience is far less than for ARM. Its a much larger risk and just not necessary; they already have their own high performance ARM based architecture.
If you think switching everything to ARM will be hell for their developers, it’s nothing compared to switching the Mac (and potentially iOS as well!) to RISC-V.
You say “at least Apple are back on RISC where they belong” like this matters. It doesn’t. What matters is how performant, reliable, customisable and risky (not in the RISC way) the choice is,.
Whether the choice ends up CISC or RISC doesn’t really matter. And intel CISC based parts have been RISC internally (in some way best of both worlds - CISC code density for better cache hits and mostly RISC performance) since the Pentium Pro (at least) anyhow…
edit:
Also… I guarantee you they will maintain the x86 platform, and track intel cpu developments internally, irrespective of what they actually ship. Just in case. They’ve been burned too many times before by reliance on somebody’s parts to not maintain the agility needed to shift as required. I doubt that will change when they’re building their own.
Fortunately NextStep was originally well architected to enable this sort of thing and macOS is essentially NextStep evolved by 30 years…