It's because AI in Bannerlord is suicidal kinda, IRL you don't survive a spear thrust to the face, meaning that, if you have a shorter weapon you have a problem, you want to rush the spearman so you can attack but you are in range of his weapon a lot sooner then he is in range of yours, and if he is good with his spear he can trust it at you before you can get to him, also he will try to keep you at range. IRL spears were main combat weapon for generations because of this very fact, but in Bannerlord(and all other M&B games) the AI will just rush you like a maniac, caring nothing for his safety, and that means you only have a short window of opportunity to use your spear before he gets too close, can't shorten it either, IRL you can grab the spear closer to the tip and fight quite effectively even when the enemy gets close(though sidearms are better) but not in Bannerlord.