Ah interesting, I wasn't aware of that. Doing some googling it seems to be a thing specific to bio and related areas, related to how NIH funding is differently structured from NSF funding. The NSF doesn't really have the equivalent of these training grants, but is more structured around 3-year projects, so students are hired onto specific projects as research assistants, or TA in years when no project money is available. Project budgets always include requests for money to support grad students, so the NSF indirectly funds a lot of training, but it's routed through the PIs proposing projects, rather than allocated at the school or department level. There's also quite a bit of DARPA funding, which is even more project-driven.