Why do Presidents swear in on the bible?

[Del Tachi]

Yeah, swearing upon the Constitution to uphold the Constitution doesn't make any sense if you have a correct understanding of what it means to swear upon something.  The idea being, of course, that by swearing to uphold the Constitution upon the Bible then to dishonor the Constitution would be tantamount to defacing the Bible.  So, to swear to uphold the Constitution upon the Constitution and then disregarding the Constitution would mean that one is defacing the Constitution, but in the prescribed scenario that obviously doesn't represent a moral dilemma to the person involved in taking the oath.