Because the nuget designers wrongly disallow creation of a nuget that has nothing. You must have at least a dependency or content. So here are some content folders, which contain nothing.
I had a legitimately need for a nuget with nothing, as I was using it as a seed from which I was going to update to a later version which contains something. So my personal choice is to not have those empty content folders, but for laughs create a seed nuget that had a dependency on "Nuget.Of.Nothing".
Install-Package Nuget.Of.Nothing -Version 1.0.0
dotnet add package Nuget.Of.Nothing --version 1.0.0
<PackageReference Include="Nuget.Of.Nothing" Version="1.0.0" />
paket add Nuget.Of.Nothing --version 1.0.0
This package has no dependencies.
This package is not used by any NuGet packages.
This package is not used by any popular GitHub repositories.