I wouldn't say it's common sense. It only seems that way to a scientifically literate person because they are so completely familiar to those concepts that it seems absurd that anyone could possibly think anything else. I would argue that on the bare basics, it seems common-sensical to think that the world is flat, firmly rooted, with the sun, moon and stars circling us because that is what it certainly SEEMS like on the surface. If you had never known any science, you would not intuitively grasp that we are in fact circling the sun along with the other 'wandering' planets. It takes some thinking outside of the box and connecting several seemingly disparate conclusions to come to that fact. If faith and religion were completely absent, intelligent people could probably figure it out fairly quickly, but it's far from common sense. Celestial mechanics are far from the day-to-day events that guide our common-sense.
However, in this modern age where all this knowledge has already been accumulated and requires little effort beyond attention to grasp, why do people still hold onto long-outmoded concepts when it is so clear to the rest of us? Simple faith. It does not matter how much evidence you put before a person who does not VALUE evidence. Scientific proof is meaningless to a person whose mental processes have settled into a mode of uncritical blind faith. Once the candle of their mind has melted into a formless lump and the flame has sizzled out, the wax hardens and it becomes very difficult to reshape it into something useful again.
That makes me think of this:
I think it is because they decided that stupid has its merits.