Even if it is, why is there no way to change the system-wide settings? All other operating systems that I know either have an explicit button "Apply settings to login screen" or do it automatically (I'm sure 99% of the consumer-level computers sold worldwide never have more than one user on them, moreso with different keyboard layouts).

Evidently you can, though in traditional macOS fashion, you exploit Apple secretly changing the setting for you if you do a magic dance.[1][2]

I've never had a reason to try it, but there's also a remark that 99% of the Macs sold probably don't need to change the system-wide keyboard layout defaults, either...

[1] - https://heylon.ca/how-to-permanently-switch-default-keyboard...

[2] - https://support.apple.com/guide/mac-help/change-the-system-l...