So quite literally, the wealthy holding the money means less money to go around to everyone else. The more they hold themselves, the more is withheld from everyone else. You can have debates about wealth distribution endlessly because it's subjective and complicated, but you can't argue that the wealthy getting richer is good for anyone except the wealthy. Trickle-down-economics has already been established as propaganda at this point.
I understand that wealth is not static in the world and it's possible to create wealth for everyone, but my point is money hoarding is definitionally the cause of wealth inequality.