Because that doesn't work across classes. For introductory classes, a grade of A might correspond to a minimum of 90%-95% correct answers on evaluations because everyone excels at the material. For upper-level classees, a grade of A might correspond to 75% correct answers.

All grading systems are essentially arbitrary.

A 95% average could be the same as an 80% or a 99% somewhere else. A 4.0 GPA could be a 3.1 or a 4.3 somewhere else (some schools use a "4.0" scale that goes above 4.0).