Sure, it’s difficult for people outside the group to reliably ascertain who actually contributed what and how it relates to the final output’s quality. Collective grading in group projects is a sacrifice of a small bit of justice for a great deal of administrative convenience - not the last time that will happen in a student’s life.

But when a system does bother with individual differences in contribution, inequality is seen as necessarily unjust. I don’t see how you can live through a group project and believe that. Some people contributing much more than others or being much more competent than others is eminently plausible! That’s how most things go!