Because mark inflation is a thing, and schools are different. There are schools that will give out A+ and others a B, for the same academic performance. School reputations don't change from year to year. So a school that inflates marks one year, will more than likely inflate them the next year. Are you suggesting that this should not be taken into consideration? That you should simply trust the relative weight of grade (relative to all the other schools) at face-value?
>"bad" schools get bad marks.
Why the quotes? There are schools that are at the bottom of academic rankings. That's a fact of reality.
>students are finding their grades marked down, almost regardless of their performance.
What numbers are we talking about here? No matter what algorithm or heuristic they chose, some unfairness was going to happen and my argument is that these cases are overstated. I'm sure they exist because we're talking about hundreds of thousands of students all in different circumstances. Having said that, in cases of egregious outcome, an appeals process would make sense. Also I'm sure many universities will take the pandemic into consideration and the fact that this was a best attempt at replicating standardizing test result without a test actually being taken.