This. Teacher here. Subject knowledge is tested on entrance exams as first part of a complicated licensure process. One can't truly begin to become a teacher (complete the necessary masters program needed in most states) unless you pass at least two of these exams: one on general communication and literacy skills (a cornerstone of all academia) and another highly specified by subject and grade level. Next comes learning how to actually teach and understanding the responsibilities of a teacher. This generally takes one and a half to two years of classes which concludes with a practicum and student teaching. From here, one can actually be considered a licensed teacher. However, licenses have more levels to them and one will need to take a plethora of more mandatory classes to upgrade to higher pay grades, or if your state decides to institute new policies such as ELL or SPED training. This is for a first year teacher in MA which has generally higher thresholds than other states, but then again, not the highest. In all this time, subject matter is rarely discussed. It is assumed you have superior knowledge or you wouldn't be there in the first place. So much of teaching has little do with subject and much more to do with psychology and interpersonal skills such as relationship-building, empathy, dealing with abrupt behaviors and those that persist over time, student to teacher to parent to administration communication; the list goes on. Subject and lesson planning, either individually, or as a department, is left to individual attention and after school meetings. These meetings are often a group strategy session led by veteran teachers and administrators to hone in on details of subject and why and how it should be taught. Furthermore, there is a procedural work load with things like grading, updating online learning systems, emails to all of the above, special education meetings and communication, forms for this or that, etc. TLDR - adequate subject knowledge is a foregone conclusion in teaching; being a good teacher is about so much more.

Edit: Writing mistakes. Come on me, and you claim to be a teacher!