AskHistorians

You were originally asking why spears hung on so long as a military weapon. Battles were not one on one duels. To ignore this seems to be fishing for some kind of answer that intentionally ignores the historical context.

Go get a 2-5.5m long x 2cm Ø hardwood pole and see how much it wiggles

Add in the fact that you don't have many skilled professional soldiers and depending on period they're probably on horseback, archers, or some other role beyond the classic hero archetype of Jet Li or Achilles.

And you have to keep in mind, a lone guy with a long spear is going to struggle, much in the same way that a cavalry rider is going to really struggle fighting on a bridge. By the time you're in that position you're losing. Spears are best used in formation, Ideally a line of spearmen with the row or two behind them stabbing over their shoulders. Short spears and javelins can act as a compromise as seen in roman legions.

Ultimately though there's a reason anti spear troops became a major part of medieval warfare. Pikes are cheap, easy, and good at killing knights