One of the rationales for the use of group work is often that it mirrors the way we work as adults—a “21st-Century skill”, which I find simplistic at best and completely wrong at worst.
Adults, working together on a project, all have the exact same goal: producing a salable product that someone will buy, which will allow each person to get paid. This is true in almost all workplaces. Even in non-profit charitable work the point is to do enough good that someone will be willing to donate money to keep that work going--and then you get paid. (The few exceptions would be in some sales jobs; jobs where each person’s commission is their own.) In the real adult world, then, every single person on the project knows the end goal and works towards it together. One weak link lowers the prospects for everyone; one particularly strong colleague floats all boats.
In a classroom of twenty students, there are twenty individual and almost-entirely independent goals: the maximizing of each student's education. Whether or not Jenny understands the math concept has, or should have, no bearing on whether Jane does. Both Jenny and Jane should be able to learn or not learn without greatly affecting the other. It is only when the teacher combines the work of Jenny and Jane, or makes them work together and grades them together that things get hairy. The goals get confused and lost. Neither student achieves the real goal: maximizing their education.
In addition, in the adult world people often specialize in one aspect of the work. The accountant handles the money, the graphic designer the appearance, the inventor the engineering, the marketers the promotion of the product, etc. For the graphic designer to sit down with the accountant to decide how to keep track of the money would be absurd; just as it would be ridiculous for the accountant to consult with the graphic designer on the product’s color scheme. Some of the specialists might need to consult with others from time to time, but each has their own work to do and goes about it independently. Usually there is some manager who makes sure the necessary communication takes place, creates a reasonable time-line, and makes sure that everyone is keeping to the schedule.
In a classroom, this also happens. Jenny might be the best at art in a group working on a project together, and so she gets assigned to do the project’s art work. As a result, she gets even better at it, and no one else improves. Jane might be better at making graphs, so she gets assigned that. She gets even better, and no one else learns that aspect of the project. Etc. Divvying up the different parts of a project works when the different tasks are being done by people who have already specialized, but children in a classroom are still supposed to be generalists. To maximize each child’s individual education, each one should do the art, the graphs, the research, etc.
It’s strange that when people like Joel and Ethan Coen (“Fargo”) or Anthony and Jo Russo (“Captain America II”) share director credits on a movie, everyone is curious about how they managed to split a single job between the two of them—people are curious because we know how hard that must be; yet, in a classroom, everyone just expects that such sharing is a natural and easy thing to do, or that it is a normal and regular part of the adult world.