Why use taught? I haven't taught anything

It is said that Mr. Wang, who has immigrated to the United States, has been teaching in No.2 Middle School in our city for 20 years. Since Mr. Wang has gone to America, it should be a thing of the past for him to teach in No.2 Middle School in our city, so I applied. Don't think about this kind of question after you finish it, which is also a common trap for setting questions. There is an important difference between the present perfect tense and the past tense, that is, you can definitely find a clear "past" time in the past tense, such as "before immigration" in this question. If you find it, you must choose the past tense without hesitation! I graduated from high school this year with NMET English score 132. As long as it is a high school question, I can still answer it. Please feel free to ask me any questions in the future! You must accept my answer. I just registered today. This is the first question I answered.