System.Linq.Queryable.dll cannot be Decompiled

Question

I tried use DotPeek to decompile the System.Linq.Queryable.dll, but it only shows the metadata like below, this assembly resides in C:\Windows\Microsoft.NET\Framework64\v4.0.30319\System.Linq.Queryable.dll. But I decompiled another C:\Windows\Microsoft.NET\Framework64\v4.0.30319\System.Web.dll assembly, it works fine.

I wonder why it happens like this ?