So, money is likes votes into the economy, and it decides what the economy produces. When a rich man decides that a house should be built for his two dogs, and that another human should spend their time taking care of those dogs, he can use his money to influence the economy to produce as he wants. The labour does not pop into existence from the void, similar with the materials for the house. It is a redirection of the economy to produce what he wants.
Money is not like mana, it does not conjure things into existence, it moves (through the invisible hand) the economy to produce what the owner desires.
Now, this does absolutely not mean that the economy is zero sum (over time). There are of course something the economy can do which will be productive and produce more goods, and there can be bad decisions. Wealth can absolutely be created by actually value creation, but also by a lot of parasitic processes(and inheritance). And the owner of money gets to controll what the economy does, you don't (barely).
A large concentration of wealth will mean that the economy at large will to a larger degree be used to produce what really rich people wants, instead of producing things the middle class wants.