FTA: “large investors may well have economies of scale. They might hire maintenance people who work for them, and thus have an adequately suited number of tasks, rather than having to contract out to workers who charge a premium for not being guaranteed work, or be better at managing properties with software, or perhaps be a more trustworthy borrower and pay less for capital.”
TL; DR they make money on rents. Not appreciation. Obviously appreciation doesn’t hurt, which is why “them owning lots of properties may increase their market power, and thus they price the homes above what is optima” is also tested.