HN user
It's part of their education to learn how to take part in the community by doing things like reviewing papers.
And this "part of their education to learn" keeps really cheap labor running science. Even postdocs are considered trainees, because they will always be learning because is part of their "job"
you are not being paid to do a job (research) you are being paid to bring in even more money
And research funding is going towards big hotel, travel expenses, and conference fees.
Grad students are the laborers
Making the business highly profitable as there is no need to hire a large number of professors where you can offer tuition waivers and cheap stipends to a larger number of people who wants to get another degree