For the longest time that was kind of the Black Panther's problem when he first joined the Avengers. Everyone at the time figured that if you needed a fighter, you might as well keep Captain America on the roster. Amazing how no one remembers any of that now.
Sentry and Adam each have their own unique stories and origins. The writers have to do a better job of reinforcing that.
They also need to stop writing Adam like he's Clark Kent. But that's a topic for another time.