Because arm64 is an architecture, not a definition of an entire SoC. When you want to run on a SoC with all sorts of connected peripherals you must support those in your OS in order to use them. Some peripherals are required to run at all. To run on any SoC you'll have to implement software to work on it.
^This. We've been so spoiled by several decades of relatively compatibility between x86(_64) PC-based systems, that it's easy to forget that that's not the norm for most architectures.
We need when macOS is intentionally obsoleted and keep that hardware running a secure OS with current updates.