The explanation depends on how deep or philosophical you want to go on this conversation, but basically you are exploiting someone’s basic need for shelter for massive profit, keeping trapped in the poverty cycle as they are having to pay rent to the landlord to pay their mortgage for them and so is much harder to save for their own house. As well as reducing supply of housing on the market, thus increasing prices and making it more innacessible.
Like imagine a group of a few wealthy people buying a town’s supply of food then selling it back to the hungry residents at a 300% markup. They don’t grow it, they don’t transport it, cook it or chsnge it, they don’t do anything that ads value, just buy it and sell it at a higher price, to the people that would have otherwise bought it for themselves. Do you consider that ethical?