Running .NET Standard binaries on different frameworks

I believe the Comparison to Portable Class Libraries section probably says it best:

.NET Standard is the replacement for Portable Class Libraries (PCL).

.NET Standard is not itself a runtime it is a type forwarding mechanism for multiple different runtimes. Therefore, it is not possible to create anything but a non-executable class library in .NET Standard, just as was the case with PCLs.

This enables the class library to be consumed by executable assemblies that target specific runtimes (.NET Framework, .NET Core, Xamarin.iOS, Mono, etc).

It is helpful to think of this in terms of classes and interfaces. In pseudo-code, .NET Standard is an interface that .NET Framework and .NET Core implement.

public interface INetStandard
{
    // Only has API definitions
}

public class NetFramework : INetStandard
{
    // .NET Framework Runtime implemented here
}

public class NetCore : INetStandard
{
    // .NET Core Runtime implemented here
}

This makes it possible to use .NET Standard with either .NET Framework or .NET Core, but .NET Standard itself has no runtime, only a set of APIs that can be shared between runtimes. You can target any one of the three with your project, but you can't execute .NET Standard any more than you can instantiate an interface.

Unfortunately, you are not the first to have asked about this and unless Microsoft makes the documentation more clear that .NET Standard does not actually execute, you likely won't be the last.