Well, darn good question;
On that same theme then why train airline pilots. Just give them a basic pilots license & allow them to fly a passenger plane full of people.
Or why require CDL licenses with passenger bus qualifications. Just give them a basic drivers license & allow them to drive a bus full of passengers.
Or why train doctors. Just give them a basic medical certificate & allow them to treat & operate on sick people.
Or why do we require police officers to train, qualify, & maintain firearm proficiency before they can protect the public. Why not just give them a CPL & send them out on the streets with a firearm.
Lots of jobs or occupations require special (re advanced) training before personal can interface with large groups of people where safety is a prime concern.
There is a right way & wrong way to handle arming teachers in crowded classrooms. If we do it wrong then it will be a difficult hole to dig ourselves out of if/when something unintended involving children's safety happens.
Lets either do it right & fully train specific qualified teachers with periodic re-training & required-interval proficiency qualifications, or don't specifically arm teachers but just open the schools to general CPL carry & let each teacher/personal/general public at that school carry under their CPL & live with the accidents, fallout, & unforeseen consequences.