Oh.
Well, I don't know any other reason why Noah would have been the only one to build the ark.
Except as I recall from Catechism Noah and his family were the only good people left on earth.
But Russell Crowe is a good actor; he was convincing to me as Noah, just like he was as Maximus in Gladiator, or John Nash in A Beautiful Mind, or Javert in Les Miserables, or Jim Braddock in Cinderella Man, or Jor-El or Robin Hood...
He's pretty good in everything.
He came off to me as goodly and righteous as Noah, even if maybe in really life he isn't that much.