On Tue, 25 Dec 2018 10:46:25 +0100, Paul Schlyter
wrote:

But if the distance is irrelevant, why not perform the division and
say 0.67 instead of 20/30? Or at least 2/3 instead of 20/30? After
all it means that the person with 20/30 eyesight has 2/3 or 0.67 of
normal visual acuity.

It's just convention.