As mentioned previously it's still in the game, but I think it's a part of the immersion that it's not common in Bannerlord. Couching lances weren't extremely common before the High Middle Ages and instead horsemen used spears just like it's represented in the game.
However, many times in battles I see AI cavalry completely miss their spear thrusts while I'm completely able to time mine correctly. So I think if that aspect of the AI is fixed, spear on horseback would be a lot more efficient in pitched battles.