In Linux they would have been separate packages, but I don’t think the App Store has dependencies so how else are they going to get the required packages installed for their ide.
[1]: Yes, I know about WSL. No, that doesn’t count.
And this is not because no one wants to write a backend to target iOS, it's because Apple won't allow it. If you want to run native code on iOS, it must be built and signed with XCode on a Mac.
I will still note that debugging and monitoring are usually not going to work cross-platform - try to log to journalfs from a Windows system (wihtout using MinGW or similar).
if you want to build ios apps installing xcode is a requirement (to get the proprietary cli tools) but launching xcode is not required
to build native mac apps, neither installing nor running xcode is necessary, as you can build with the smaller cli-only tools which include only mac os headers/libs