Because the teacher is the one in the classroom guarding the door.
Is that how it plays out in your mind? What makes you think that's how it plays out? What about the teachers evacuating the students?
The bottom line is that no solution is perfect, and doing nothing while insisting on perfection is fruitless.
Doing this is more damaging than doing nothing at all.
In your paranoid mind. Some districts already have armed teachers, with no problems. Scared yet?