Why can't I get the count of enums during compile time?

I have asked How can I get the number of enums as a constant?, and I found out that I cannot get the count of enums during compile time, because C# uses reflection to do so.

I read What is reflection and why is it useful?, so I have a very basic understanding of reflection.

To get the count of enums, I can use Enum.GetNames(typeof(Item.Type)).Length;, and this happens during runtime using reflection.

I don't see any runtime knowledge needed to get the count of enums, because as far as I know, the count of enums cannot be changed during runtime.

Why does C# have to use reflection to get the count of enums? Why can't it do so during compile time?