M1/AArch64 is the future of Mac hardware, this is AWS following the path. Devs targeting the macOS desktop should be focused on M1 native exclusively unless long term support of X86_64 Macs is part of their product's support options.
I have to assume the primary use for these VMs is to build iOS applications not Mac apps. For that use case the CPU architecture used to run the build doesn't really matter since the end product is an arm64 iOS app regardless.
Sure, you can use it for testing and building Mac apps, but the number of people doing that is minuscule compared to the number of iOS developers (unfortunately).