ArcGIS Pro generates the legend based on your symbology settings for the layer. Google Earth doesn't work like that, so the exported "legend" is just an overlayed (semi-transparent) image. To see it, you need to expand the "Legend" folder and choose a radio button for one of the positions on the screen.
I hadn't tried exporting a legend from a Layout before, and frankly, I think it looks like **bleep** in Google Earth. I think a better solution would be to take a screenshot of the legend in ArcGIS and manually overlay it in Google Earth. Then you could better control how it looks.
