Because you can't use .NET CoreCLR as it is on Android. You will need to modify it in order to be able to compile it for Android. Here is a discussion about an attempt to modify .NET CoreCLR in order to be able to compile it for Android https://github.com/dotnet/coreclr/issues/1097

On iOS the situation is even worse you will need to have an AOT compiler for C# in order to be able to ship iOS apps in the Apple Store.

You are making a confusion here. Unity doesn't use Xamarin at all, they have an old license (like really old, since before Xamarin even existed) for Mono for iOS and Android.