I used to hear a lot of arguments about attacks per round, especially in 1st and 2nd Ed, when rounds were a minute long.
In every edition, a character is presumed to be swinging, thrusting, parrying and dodging more or less continuously when in melee. Most of these attempts are to keep your opponent off balance, to try and create an opportunity for yourself. Once per round you actually get such an opportunity, an opening in their defense that lets you get a shot at something other than their blade or shield.
When someone drops their guard to do something like cast a spell, drink a potion or turn and run, they're creating such an opportunity and handing it to you.
As you get better in melee, you find that you can slip a shot in through smaller and smaller openings in the opponent's defense.
When you think about melee combat this way, it's easy to see why subsequent attacks would have penalties, and why those penalties would mount. Your window of opportunity for 2nd, 3rd and 4th blows keeps getting progressively smaller.