Good question. The use case we are primarily seeing with forking projects is a bit different than the typical GitHub/GitLab fork use case.
We see it being used in the teaching of courses. The instructor creates a project with all of the materials necessary for the course; the students fork this project to get their own copy of it for doing their work in the course. In this scenario, we do not expect contributions from the fork back to the source project.
Overall, the guiding principle in the UI has been to support features and use cases that are different from GitLab, and defer to GitLab for things that are well-supported there. This has the unfortunate side-effect that it leads to an erratic user experience, especially for power users that want to take advantage of GitLab features, but it allows us to dedicate our resources on the differentiators.
We are revisiting this decision and intend to duplicate some GitLab functionality to provide a smoother experience to our users, and we would be happy to hear feedback, especially about things that are in your daily workflow that you would like to have better supported in Renku.