"Insurmountable" means "too great to be overcome"?
LearningWithInternet
According to the Oxford dictionary, "insurmountable" means "too great to be overcome".
I don't know why it isn't "too great to overcome". Because when describing a book, I can just say "this book is easy to understand.", instead of "this book is easy to be understood."
The Oxford dictionary uses the passive voice here is absurd to me. By the same token, shouldn't it be "too great to overcome"?
Well, my question is, is there a general rule to this? Or I just got confused, this question actually shouldn't exist?