The reason teachers don't have to learn them all, is because they already have to learn their own subject in a level much higher than the students. Primary school teachers have to teach them all, because it's not that hard for a grown-up to be way above the level of an average six- to twelve-year-old kid. Also, you don't have to be good at all the subjects. Most school systems offer pupils some sort of choice in what (package of) classes they want to take.

One teacher teaching one subject knows much more about it than you will have to therefore is more capable to explain it the basics or more. As a student you have to learn all of it so as to have average knowledge for future (basicly you will not look like idiot)

Presumably, if you will learn only one subject, you will know at max as much as your teacher.. well, I personally really want to be more educated.

There used to be something known as the ideal of liberal education. The idea was that a student would be well-versed in the principles and literature on a wide range of subjects, and that there would be what could be called an overall synthesis of this knowledge. Such an ideal is supposedly supported in the criteria of "general courses" in modern institutions of learning. Unfortunately, this is a mere chimera. General courses are more of an excuse to line the pockets of universities. From any reasonable point of view, they only serve to be an unwelcome financial burden on students. Perhaps such courses can do the favor of "supplementing" one's GPA, but it would be more sensible, at the very least, for general courses to be optional at our colleges.

It may be instructive to ask, where exactly did this ideal go wrong? Such a question leads to some rather extensive disquisitions on educational philosophy and pedagogy itself. However, I will give my own version of the answer. In the first place, it went wrong because today's students are not making any "grand synthesis" of principles from a wide survey of subjects. Rather, students are instructed (practically coerced) to be "well-rounded." Unfortunately, being well-rounded in today's school systems often means having at best a superficial understanding of a wide domain of disparate principles, with no obvious connection in the mind of the student. This lends itself to increasing specialization in our universities, where there isn't really any sort of cross-fertilization and communication between academic disciplines that leads to a greater synthesis.

In today's schools, instructors only need to have the requisite specialized knowledge in their discipline of interest. However, this tends to undermine other pertinent considerations outside that instructor's area of expertise (e.g. the quality of the methods of instruction and the teacher himself). This is not to say that one cannot grasp a wide variety of principles from different disciplines. However, for this task, one must often rely on self-teaching, which can sometimes be a rather arduous affair (especially when reviewing needlessly obfuscatory and abstruse texts). So perhaps part of the answer is that the ideal of liberal education is to a large extent dead in contemporary society, and it is often replaced with a shallow notion of "being well-rounded" that so frequently (and rather distastefully) pervades our schools. Whether such a problem, if it is indeed a significant one, can be potentially resolved is beyond the scope of my answer. I will say, however, that certain monetary and bureaucratic incentives seem bent on making sure that it is not resolved any time soon.

They should teach them all because it gives you a better education! If the single teacher cannot teach all of the subjects then they should have a specialized teacher for each subject! Just like in High School! Not learning them is not an advantage for you're future!