Indeed, if black plays it well, he gets at least equality. But the good thing (bad thing?) about humans is that they're not perfect, and make mistakes (or just don't know theory). Theoretically the Morra gambit is refuted, but I bet if you try it 100 games against 1600 rated players you can score about 80 points

(provided you do play it well and you're better than your opponent)
I had another quick victory, I'll post that one soon.