Because they don't possess the personality needed to interact well with young people. If a person doesn't like kids, doesn't enjoy being with them all day long, doesn't look forward to teaching them, doesn't accept their immaturity and want to help them become more mature, can't stand constantly answering questions, can't accept individual differences (race, ethnicity, gender, religion, etc), can't cope with special needs (hyperactivity, behavior problems, and so on), then that person will never be a competent teacher.
Because they don't possess, exhibit, use, and treasure enthusiasm, and, so, they are truly boring to most of their students. Ask any kid at a middle school, junior high school, or high school in your community what they dislike the most about their teachers, and, I guarantee you the answer will overwhelmingly be that they are boring. And you know something, Tyler; the kids are right. Most teachers are insufferably boring in how they teach. Enthusiasm is a sine qua non for all competent teachers.
Because they don't know how to get concepts and ideas across clearly to their students. They don't possess the knowledge and skills needed to help students learn. They just don't know what to do and end up quite often being frustrated and saying something like, "Oh, those kids just can't learn this stuff." That's an expression equivalent to defeatism and incompetence. If the learning material is age appropriate and part of the accepted curriculum, of course a normal, healthy student can learn it. It isn't the student who is at fault; it's the teacher who doesn't have the competence to design lessons, activities, and programs to help students learn. The reason for this is that many teachers tell students but don't show and teach.
Because they can't manage and control student behavior. Teachers daily face challenging disciplinary and behavior problems. If a teacher can't effectively handle these problems, that teacher will never be a competent instructor-never! In this case, the incompetence is in not knowing what to do when a disciplinary or behavior problem presents itself because the teacher hasn't thought out a personal Educational Philosophy for Control of Student Behavior. Every teacher needs to do this to harmonize his/her personality with methods of discipline. I explain this in detail in my book.
Because many teachers don't manage classroom time efficiently. I devote an entire chapter to this topic: "Wasted Time - Inept Instruction (Euphemism: Teaching Mistakes). How can anyone consider a teacher competent when that teacher tries to teach over the noise of unruly students, doesn't know how to quell effectively unnecessary noise at the change of a classroom activity, and allows students to talk whenever they want. This inability to control noise leads to as much as 25% of each class period being wasted. Many teachers can't even control the time at the end of class when students get ready to leave and waste the ten or fifteen minutes left.
Because many teachers can't effectively control group learning. One of the most effective ways for students to learn is to interact with each other, allowing students to help each other learn in groups. Sometimes, students have just the right words and explanations to help a fellow student understand a lesson. However, most teachers don't control student groups effectively and so waste tremendous amounts of instructional time.
Because many teachers don't have high enough academic and behavioral expectations and standards. In other words, many teachers don't challenge their students enough academically and don't expect them to learn to the level of their potential. Teachers must project an attitude of high expectations to motivate their charges adequately. Most teachers don't even understand this concept and need to learn it themselves. Not putting it into effect in classrooms is indicative of ignorance and incompetence. In Chapter Three, I wrote a seven-page description of the most important strategies used by teachers who truly understand how to teach high academic and behavioral standards. Teachers, you've never seen anything come close to this practical list of how to teach standards.
Because some teachers don't have a sufficient knowledge of the subjects they teach. They don't! They are assigned to teach a subject they don't know adequately or they don't even like. Many teachers are teaching subjects and they don't have either a major in that field or a valid certificate to teach it.
There are other reasons as well, but the few I mentioned are really significant ones, aren't they? Now, what are the reasons for these incompetencies and why do school systems allow these incompetent teachers to remain in the classroom? Well, the first part of the question can be answered easily. Students learning how to teach are not being prepared adequately by schools of education. You know who should teach prospective teachers how to teach? Not education professors! No! Excellent, experienced, current and retired teachers who know what a classroom is all about and who have a love for kids and teaching in their hearts should teach candidates for teaching. Give me proven experts at teaching young people, a group of twenty teacher candidates for a year, and I know we could do a much better job of teaching them how to be good teachers than any school of education in the country.
Answering the second part of the question leaves me with a heavy heart. The reason is that most school districts don't effectively monitor and evaluate the progress, competence, and teaching skills of new teachers. The procedures to do this are woefully inadequate and rarely result in new teachers being dismissed if they are incompetent. Teachers new to the profession learn more about teaching from their own personal experiences the first three years on the job and from other, experienced teachers than they do from any program presented by the school district they work for. School districts don't really know if a new teacher is mediocre or, worse yet, incompetent so they grant tenure because they need a body in the classroom. There is a tremendous shortage of teachers throughout our country today. Once tenure is granted, it is virtually impossible to dismiss a teacher on the basis of incompetence.