In my state, firearms instructors must attend an eight hour recertification class every three years. I will go to my fifth recert this year. Every time I have gone, we have shot the state handgun and shotgun qualification with a required 90% score, shot the FBI Instructor Bullseye with no minimum score, and then listened to the same speech about how "next time" they will be making instructors test out of things like reloads, malfunctions, etc. It never happens. It probably never will happen. It's like mandatory, in-service physical agility: if you were willing to accept the cost of instituting it, there would be long term benefits. If they said tomorrow that all recerts must post a 270 bullseye score, meet standards in various skills, and pass a written test or something, they would lose 30%+ of the instructors right away.
The fact is, though, that at least here, the state academy's (which is the certifying board) customer is the agency, and the agencies want certified instructors, so the academy gives them what they want.