Teachers typically assign different types of homework for individuals vs collaborative groups. One reason is because of ease and another deals with fairness.
It is easier for teachers to assign homework to individuals because the students only depend on themselves to finish the assignment. Since only one person is working, the assignments are normally smaller. Students can individually ask for help and students cannot copy off a group member. In collaborative learning, students depend on others for help, which is a great benefit, unless they take advantage of that.
In collaborative learning, homework consists of larger assignments. Too often collaborative groups do not work fairly, and one or two students do the work. Collaborating outside the classroom is also a struggle, as students may not be able to meet. If done properly, groups are proud of their finished assignment and have learned important skills for working with others.