This study is one in a trend of studies that have been showing that the rich are lacking in attributes that are valued by the majority of people while possessing a surplus of those considered to be negative traits by the majority. I.e., rich people are unkind, less generous, less empathetic, and now, in this study, they* are shown to express less gratitude. And, the wealthy are more self-centered, behave unethically, are narcissistic, and feel entitled.
https://www.theatlantic.com/ideas/archive/2021/04/does-wealt...
* In capitalist societies, wealth brings power, so I am treating 'rich' and 'powerful' as equivalent.