You know most millionaires are new millionaires because they are usually in the later stages of their careers, commanding higher salaries and gaining investment windfalls so that does not constitute the "rags to riches" meme that I think you are associating with that idea.
To answer your question, people generally have a grudge with the rich getting richer because the poor and middle class are getting much poorer, and seeing their incomes stagnate in relation to the great wealth being accumulated by the rich and super rich.