[gtpedrosa]

I think the Makani documentary does a good job explaining the problem in less than 2 min [1].

[1] https://youtu.be/qd_hEja6bzE?t=392

Regarding the size x quantity, I believe the reason lies in where the best wind is, which is usually high above the ground. The roughness of the terrain and obstacles generate shear and turbulence, which translates into more stress for the components. The higher wind has a more uniform distribution across the rotor and is higher in magnitude than in lower heights. So for small wind turbines to have access to the best wind, you would have to build expensive structures to reach there, making it infeasible. Hence kite approaches like the one posted and Makani (with different principles).