Just from looking at his gallery, it's his style. He doesn't fully render the legs/feet but he does the chest/shoulders/head. My guess is that it's because the head is the focal point of the character design.
Can't really judge skill based on something like that. I don't see one painting of the back of the knee. Could say it's because he can't but probably because it's not a priority.
"Astronomy offers an aesthetic indulgence not duplicated in any other field. This is not an academic or hypothetical attraction and should require no apologies, for the beauty to be found in the skies has been universally appreciated for unrecorded centuries."