For Julius Caesar:

Why didn't the enemies, Antony and Octavious versus Brutus and Cassius, simply kill each other when they met?

There were so many rules about civility in war in Roman times. Many times they attempted to sort out a disagreement before the war was even started by having a "parley". This was a meeting to discuss a potential truce or other issues. It was considered very bad form to attack your enemy while in a parley, and Romans took those matters of pride and civility very seriously.