I ran into this too. Both the Simplify Polygons tool and simplifying the symbology in general didn't solve it.
Instead, I re-imported my original files and only made one change (changed the symbology), used Layer -> KML, and it worked. I'm not sure what I clicked the first time for the polygons to not show up in Google Earth as a KML. I'm writing this so that future people with this problem could try this method too.