Why is the English language teaching as difficult for EFL teachers as learning the English language for EFL students?

Every language learning process takes years to acquire mastery over it. English is an easy language and this should be felt both by the teachers as well as students.

It's never been any difficucl to me 

With practice comes perfection

I think it is easy to teach EFL if the teachers are skilled in methods of teaching. For students, the fact is that children are endowed with a process (blueprint), innate template, for language, this process aids the child in the task of constructing grammar. As a result, children are neurologically prepared to acquire all aspects of grammar in an easy way. According to that, Noam Chomsky says that "Language learning is not really something that the child does; it is something that happens to the child placed in an appropriate environment..."

In my opinion, it depends on the teacher's skills.

because as foriegner in English

Teachers should be models for thier student. the perfection of the teachers is more difficult than acquisition of what students can get

If you Love English and understand it well as a teacher, you will most certainly pass it on smoothly without any detours .