Because ASP.NET Core is a framework that is installed with .NET. It isn’t a set of Nuget packages.
Why? .NET Core 1.x tried making everything a Nuget package and it was a disaster. ASP.NET is big which meant a huge number of dependencies, easy to create conflicts, long restore and build times.
It was much better to just include ASP.NET Core in the box with .NET and simplify it down to a single reference. And most people use the web SDK which does it for you so they don't even need that.
Why? .NET Core 1.x tried making everything a Nuget package and it was a disaster. ASP.NET is big which meant a huge number of dependencies, easy to create conflicts, long restore and build times.
It was much better to just include ASP.NET Core in the box with .NET and simplify it down to a single reference. And most people use the web SDK which does it for you so they don't even need that.