There is a simple explanation to this:
When Collect Earth starts, it generates a KML and a KMZ file that contain the placemarks of the plots from the plot location files selected.
Collect Earth uses a Operative System command to open the KML file ( which in its turn opens the KMZ file ). It basically tells the Operative System to open the KML file with the default program (which is expected to be Windows)
In order for this to work it is mandatory that Google Earth is the default application that opens the KML files.
It might happen that you install ArcGIS (or other software like Garmin or TomTom GPS or mapping software ) after you installed Google Earth. This software would typically set itself as the default applications to open KML files.
Solution ( in Windows ): Go to the folder where Collect Earth has generated the KML and KMZ file, normally it should be :
C:/Users/USERNAME/AppData/Roaming/CollectEarth/generated
Now right click on the file called loadApp.kml and in the context menu choose Open with and then Choose Default Program like in the image attached. Finally in the dialog that is shown choose Google Earth and keep the "Always use the selected program to open this kind of file" checked.

