Cannot find `ZipArchive` in the “System.IO.Compression” namespace

Question

My question is related to I didn't find "ZipFile" class in the "System.IO.Compression" namespace

But I have referenced the DLL's for my 4.5.1 webforms project:

Properties of my project give me: Target framework: .Net Framework 4.5.1. and the web.config:

<compilation debug="true" targetFramework="4.5" /> 

What am I missing?

The solution was to manually reference the assemblies in the web.config But why? Why wasn't the checkbox in the add reference dialog insufficient?

<assemblies>     <add assembly="System.IO.Compression, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />     <add assembly="System.IO.Compression.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" /> </assemblies> 

Taken from this answer to "Installed .Net 4.5 but can't use ZipFile class in Visual C#"