Because even if big, demand is limited by what people can actually pay. You can price-fix and collude with other big home-owning landlords to milk your buyers,

Because even if big, demand is limited by what people can actually pay. You can price-fix and collude with other big home-owning landlords to milk your buyers, but to a point. Beyond that people just wont rent and move away, downscale as much as they can to smaller appartment, find roomates, or try to move back to their parents.