Explanations :

https://www.abondance.com/20190917-40727-google-modifie-les-conditions-daffichage-des-etoiles-dans-les-serp.html 

Note that Bing started the opposite to post the ratings only a few months ago!

As a reminder, here are the market shares :

https://gs.statcounter.com/search-engine-market-share