Can't Add .Net 4.6 Reference

I do this (Visual Studio 2015 Update 3):

• Create a new ASP.NET Core Web Application (.NET Framework)
• Select Web API -> OK
• Right click References -> Add Reference
• Browse to a DLL compiled with .Net Framework 4.6
• Error:

The strangest thing is that I CAN reference .Net Class Library assemblies if I add them to the solution. For example, I created a .Net 4.6 class library and referenced it from the MVC app mentioned above. I was then able to compile and run the MVC app. So, why I can't I just add DLL references directly?