Im going to assume you are ignorant and not malicious. There are TA's and student teachers which are phd students. Yes, they are "students", but they typically only teach after they have completed their oral examination and achieved an A.B.D. (All but dissertation) status. In the humanities the oral exam is to read hundreds of books in your main field and two sub fields and have 3 different tenured professors grill you for hours about all three different topics. The people who pass these exams are no longer mere "students". A TA just grades papers. A phd student who is teaching a class actually can compose a curriculum and make the lesson plans for an entire semester. This takes hundreds of hours of work. Tenured professors may not share lesson plans, or really be involved at all, so you do things from scratch. Effectively they are professors and they are teaching classes on their own.

Why would phd students do this when they are on the clock for time to degree and completing their dissertation? Often they have to make ends meet because phd stipends are laughable. Make money on the side you say? Nope, if you do, and the university finds out, they dock your stipend to match. So you have to live in NYC on $31k for seven years, with shit healthcare, and little to no dental or other care, which is basically impossible. Something has to give, and its usually the student's own health. Also, all of this while navigating the machiavellian clusterfuck that is academia just because you want to be a teacher and contribute to the next generation.

Source: My wife is a phd student and the shit they have to endure is insane. Watching from the sidelines is very frustrating, and I hope the modern academic system gets an overhaul soon.