The trouble with using a method like this with an average is that mean ≠ median, and for the vast majority of schools and subjects, marks can exhibit a positive or negative skew rather than perfectly symmetrical data. I found this myself while working on the raw atar calc, when I initially included the average aligned/scaled mark conversions in my modelling it was significantly different to the percentiles, and once I removed them it made quite a significant difference to accuracy - and that’s on the scale of an entire subject cohort, singular schools are significantly more prone to outliers. Band 6 counts are far from a perfect method either (and in an ideal world everyone would provide multiple years of data as that would improve accuracy by a fair bit), but the way they work off a specific percentile is a significant advantage.
A better data point I’d like to see more people include is band 5 statistics, which I’ve found many schools include in their annual reports, as they would aid significantly in ATAR estimates where people have ranks in that range.