How do you know?
Also: it's possible to make an "anonymous" donation and let your friends and family know you're doing it. That way, to the people you know and want to impress you seem both like a generous philanthropist and a humble private person.
My point is, it's possible to read any action in a bad light and assign a bad motive to it. In my view giving money to worthwhile causes is good, even if you're doing it to rehabilitate your image or make yourself look good
You asked what could be the reason why "are most people in 2019 so convinced that Bill Gates is "the worlds greatest guy?"
I'm pointing out it might have something to do with a $50 billion philanthropic organization.
http://www.openculture.com/2019/01/how-alfred-nobel-read-an-...
The list is long. Carnegie, Rockefeller, Vanderbilt, Pew, Kochs, ...
Then employ your family members to run the foundation and pay them a huge salary.
I can’t validate this, but I recall the article.