This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
That is because it doesn't relate. The two are completely different.
When measuring pH you are looking at how well the acid disassociates in
solution, but grams/liter is a measure of how much acid is actually
present. There is no way to compare the two.
In theory curves could be built to compare g/L to pH, however the
relationship changes from grape to grape, year to year, fruit to fruit
and of course the particular blend of acids that are in the wine. This
constant changing and unpredictability makes it impossible to relate pH
to g/L acid.