I think you are mistakenly generalizing on a single instance.
As far as I am concerned may be used in contexts where it has approximately the same effect as saying in my opinion; but it does not mean in my opinion.
Examine the actual construction of the expression as far as X is concerned:
as far as means to the extent or degree that
X is concerned means X is involved or relevant
Consequently, as far as I am concerned only has the effect of saying in my opinion when it is used to introduce an opinion.
As far as I am concerned, Bob’s a jerk. —This means “My concern in this matter does not go beyond recognizing that Bob’s a jerk; I am not interested in any subtler analysis of his behavior.”
But in other contexts as far as X is concerned takes on quite different meanings:
As far as the economic problems here are concerned ... —This means “To the extent that our discourse involves the economic problems here [I have this to say] ...”
Similarly, as far as I know means to the extent that my knowledge reaches—that is, “I do not claim to know everything about this topic, but what I do know is this ...”