- Teachers are more enthusiastic and interested learners. If you have a group of 30 teachers, you have a group of 30 people who probably want to be there. If you have a group of 30 high schoolers, you probably have 2-3 who really want to be there, and at least one who would rather be *anywhere* else.
- One teacher reaches hundreds of students. Teaching 30 teachers about science, and what scientists do, potentially reaches thousands of kids over years. Working directly with students reaches those students, and some of their peers.
- Teachers can tie in new concepts into the curriculum much more effectively, because they know what else is actually covered.
This is not to say that I think we should stop running student-directed outreach in the slightest. Summer science camps are a fantastic thing, and many of my peers went into engineering because of similar experiences. It also gives kids a chance to explore an area that may not be feasible for their school to have a program in, due to demand/budget/classroom space. However, teaching teachers is a great way to reach a wider audience. For a field like MSE, which many students don't even know exists until partway through college (and some not until they've graduated and gotten a job), having potentially interested kids know the field exists is a pretty good start.