Whay asp.net core's Startup class is not an inteface or abstract calss

This is in regards to the design principals behind the Startup class explained here:

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/startup?view=aspnetcore-2.1

I understand that the class needs to include methods like ConfigureServices or Configure.

Why CreateDefaultBuilder(args).UseStartup<Startup>() does not mandate any base class or interface for better readability?

With this design approach, someone must read the documentation and know about the magic method names like ConfigureServices or Configure.

If this is part of a new class design mindset, then where can I read more about it?