Entrepreneurs and Wealthy People

Seems that people like Bernie Sanders and Elizabeth Warren are always calling out successful and wealthy entrepreneurs, and  business people. I guess it stems from them making their money by providing a service, a product and not an agenda or social movement; or they don't feel that they pay enough in taxes. I really don't have a good answer to why they or anyone else seem to equate one person gain as somehow making someone else poorer for it. So let see if we can make heads or tails out of this rabbit hole.