Uh, maybe because IQ tests didn't exist in their day. Just sayin...
You have to remember that, in the late 1700's when the Constitution as being written, only prominent, educated men rose to become leaders in their colonies/states. Most were landed gentry -- gentlemen as the word was meant then. There was a code of honor among gentlemen, and the Founders' expectations were that Congress would be comprised of gentlemen of honor who would always answer to the people (house) and the states (senators). Like all human beings, they could not foretell the future very far beyond their own times.
In hindsight, I wish with all my heart the Founders had written term limits into the Constitution. They didn't because, in their day, no man wanted to make a career of being in Congress. They all had families, businesses, land and farms to return to, usually significant distances and many days travel from Philadelphia and New York (when the early government resides there).
I agree completely, they could only go by their experience and "world view" at the time. In addition, on your first point, not only were the representatives educated and honorable, I don't think the Founders could ever imagine a citizenry that would ever become so ignorant, greedy, and self-centered. I think it is a fair statement to say that citizens of those times would never be duped into voting for representatives that are so reprehensible.
On your second point, again, based on their experience and world view, they didn't imagine that term limits would be needed. Exactly as you say, the honor of being a representative was never viewed to be a "career" position. They would take a turn if called upon to SERVE, then return to their prior situations.