It's an issue with practicality: a smaller wind turbine requires a similar amount of maintenance as a huge one, while the latter converts the energy more effici

It's an issue with practicality: a smaller wind turbine requires a similar amount of maintenance as a huge one, while the latter converts the energy more efficiently due to greater rotor span. Then you are not going to build a sole turbine† in a middle of nowhere, because it requires a transformer /DC inverter and a connection to grid. These things are costly and best amortized over large farms. Same goes for maintenance: there's periodic maintenance for various systems and much of the cost is getting specialists and gear there and back. (†) There are cases when you'd use turbines of grid, like with remote mines/outposts but these are rather special.