To answer the "why" folks may think this, you and I are considering the general equilibrium impact of the policy, but the vast majority of folks think in terms of partial equilibrium/first order. And on a first order, the "jobs" that folks holding these student visas will be vacated and thus "available" for domestic students. Such nativist views typically fail to grasp that said jobs may simply not be available due to companies sending work to the talent, not just to the trained.

Myopia is a terrible thing to live in denial with, but unfortunately such as things are.