The only thing wrong is you are comparing a $1700 it to a $6000 and $10000 lens that are at the top of their class. Really IMO there not in the same category.
No doubt they have better build quality, light transmission and wouldn't they start with more light since they have a larger front element.

The 100-400mm II might be a better comparison. Then I think you look at number of elements, light transmission of the each element and build.