Yes, I know they have probably armoured horse with bigger mass and better armor and whatever... but as unit vs. unit, they are worse and have bigger upkeep.
You point out the reason they are more expensive, then ignore it. Lancers have the mount with the highest mass, and combined with a very high charge bonus means they are one of the best cav units for charging, so are very useful for flank or rear attacks against pikes, and frontal attacks against sword infantry

They have no shield, so they are not meant for cav v cav combat.