You appear to be encountering a known limit when converting line and polygon labels to KML.
This is documented as Technical Article 43000: Labels do not appear in Google Earth after converting to KML:
This is a known limit. Google Earth only displays labels for point
features (placemarks). To display labels for line or polygon features
in Google Earth, midpoints or centroids for those feature classes must
be created as a point layer in ArcMap and imported as a separate layer
to Google Earth.
Workarounds using Advanced level, as well as Basic and Standard level licenses, are documented in that Technical Article.