Why, those who are passionate about teaching and education should not join academics?

There is no definite answer to this question.  Being passionate about education does not mean that one should follow the academic track or not.  It is about the ability of the person to be a good academic.  Of course you dont want any academic to be leading others without passion.  However, passion is note mean that one should be an academic.  To be an academic you need passion but you also require sound theoretical skills and the ability to work as an academic.  Academics are normally passionate with the requisite academic skills.

Maybe some official agencies do not want it

This is a debatable topic. There could be as many reasons against it as many in its favor. I believe in survival of the fittest.

While not inclusive of all possible reasons:

1) can get pretty political at the top

2) pay is not great for many low demand disciplines like humanities and social sciences 

3) there are far more qualified PhDs then there are tenured positions available for many disciplines

4) not all students are fun to teach

5) you may not be able to research what you want, funding may only be available for certain topics

6) did I mention the pay is not great?

Again, I do not seem to understand the question fully. What does the asker mean? If a teacher is good at academics, why shouldn't they join?