After I played as a spearman in the last set of finals, I don't think spear is weak against cav, if you know how to use it properly. To me it looks like due to the lower amount of players in Skirmish, compared to Warband battle, it's easier to be punished while playing with a spear by enemy team, so people don't even try to play with one, even though it's still very doable and if your team has inf advantage which means you get space to stop cav, I'd say it's very effective vs cav lol.