The problem is you're thinking of Gangplank as a carry - he's not. Despite what you might think, he simply can't put out consistent damage like other carries. He's too fragile, has no gap closer, and no way to stick on his target like other melee DPS do. What he does have is good movement speed and a spammable, hard-hitting poke (Parrrley). Early game he has a global ultimate to help ganks suceed, and he can deny exp and gold from his opponents.
So what we learn from this is that Gangplank is support DPS. Similar to Teemo in that you don't pick him for his damage output, but his utility. Since it's more common that you'll avoid autoattacking in teamfights in favor of staying at range to Q spam (to avoid dying), while using your deny to buff your team, I think that proves the point. Plank's job is to soften up the enemy or to burst down squishies while his ultimate helps slow the enemy team down as your team pounds on them.
Obviously, his use as a support DPS is questionable when you pair him against many other support DPSers out there, and this is why he's not really thought of as a strong pick for ranked play. Not that he's a useless champion, but I hope you see what I'm trying to get at.