Then if you can focus 20 times the light on it you're generating slightly less than 20X the power
No, actually more like 80X! Because it converts the light to electricity at 80% efficiency instead of 15%-20% for un-concentrated. This is due to the extremely steep temperature gradient between the super-heated front-face diode receiving the sunlight and the water-cooled electrode behind it. (I'm sure somebody else can explain the physics better).
The point being, say you have a rooftop in a city and want to make power - in that case, density matters.
And if this were stupid-expensive, it would be a research project and not a product.