It is easier – even quicker, once you have the habit – to say In my opinion it is not an unjustifiable assumption that than to say I think.
I want to say this is because this is how we've been trained to write, regardless of whether it was purposeful or not. We learn to write from school. There is no set grading system on writing between teachers, but what tends to get you a better grade in a class? It is filling up the required page length with fluff that is sufficiently tangible as to feel prose-ish. If teachers were truly concerned about fixing this, it should be reflected in the grades they give.