Most American etiquette “rules” were established in the late 19th century. Prior to that, American manners were indeed atrocious by standards of the time, and probably today. “Grab, gobble, and go” was the standard method of of eating, and you were expected to push aside those who were too slow and who got in your way. People would loudly slurp up soup, stick their hands in front of others to get something from the table, talk loudly with full mouths, etc.
The main reason for all the rules during this time was to control your behavior in public. People were supposed to be “refined” and “reserved” and “grab, gobble, and go” was frowned upon. The rules all stressed self-control – you were in control of yourself while you ate, eating slowly, talking (not with your mouth full, of course) between bites – it was “please pass the bread” instead of grabbing a slice.
Part of this self-control was sitting upright at the table, back straight. Keeping the elbows off the table was related to this. Though it was easy and convenient to eat with elbows resting on the table from time to time, but you would no longer be seated in the proper manner. You were giving up self-control, which was considered a social blunder.
It was very logical within the society at the time, according to social norms.