First, school knowledge is theoretical. This is because everything we learn at school is very theoretical, all for exams. For example, what we study in high school is to cope with the college entrance examination. However, the content of university study is theoretical and seldom practiced, so more practice is needed after entering the society, while what is learned in school is theoretical and unnecessary at all. Of course, not all majors do. For example, some science and engineering majors need more theories to support their own practice, so in many cases, they will complete their tasks according to their own theoretical knowledge when they come out.
Second, it is more about life, entering the society, facing your own life more, and solving problems realistically. Theoretically, he can't support us to finish these things, so most of the knowledge in school is unnecessary. In society, you will not be asked to explore the necessity of two things, but only how to do it. And many times we go to school just to broaden our knowledge. These things just exist in our minds and cannot support us to finish our work.
Third, when facing practical problems, because when studying at school, he won't tell you how to solve a problem, but what methods you can use, but what kind of theoretical knowledge you need for this major. Therefore, when we face practical problems, we may not remember the knowledge taught in school at all. Moreover, what some schools teach is superficial, which makes us completely unclear when applying these theoretical knowledge.