NXLog for Windows - include_stdout not working
If you are referring to the NXLog User Guide I think the very first section should give you a reason for this if you are using the NXLog Community Edition:
Though most of the content applies to all versions of NXLog Community Edition and NXLog Enterprise Edition, this guide was written specifically for NXLog Enterprise Edition version 3.99.3035. Some features covered by this document may not be available in earlier versions of NXLog and earlier versions of NXLog may behave differently than documented here.