Bucklers were designed to be used for fencing swordfights with people using light swords, not for medieval battlefields where guys are running around in plate armour and using linebreaking weapons like heavy axes and 2h swords where heavy shields were effective.
They were only added to the game as a novelty, so people could RP some kind of italian duelist if they wanted, you can't expect them to be balanced to par with a huscarl shield or norman kite shield.