Because for some reason, he is at least proficient in only one Martial art called The Quack. I know, that sounds lame, but that is the name of it. It's like hearing a normal duck saying "QUACK QUACK".
Sentry is not a expert in hand to hand combat, but an average combat fighter. That is not good when Sentry fights someone like Hercules who is a master combatant.
Oh yeah. Take a look picture of Howard the Duck. The Guardians of the Galaxy is trying to support him, but he makes that rejected face expression. I mean, what's wrong with him?
Comments
Sentry is a Dark Avenger (sometimes) and has the power of a million exploding sun.
HTD is just a duck
It feels like Howard is just a duck who chills on the pool.