In order to roslyn analyzers/source generators to work they need to placed under Assets/ folder (https://docs.unity3d.com/Manual/create-source-generator.html)
The reason for that is that Editor will not import assets outside `Assets/` folder whence compilation pipeline will not include the analyzer.
The fact that the analyzer is included in the .csproj does not affect this because Unity does not use the .csproj in the build.