Why do Bassanio and Antonio borrow money from Shylock, a moneylender?

In act 1, scene 1, Bassanio petitions Antonio for money in order to fund his trip to Belmont. Bassanio explains to Antonio that he has been careless with his money and is in debt to numerous people throughout Venice. He goes on to tell Antonio that there is a wealthy heiress named Portia who lives in Belmont and has many suitors. Bassanio explains to Antonio that he believes he will be able to win Portia's heart and marry her. Marrying Portia will make Bassanio a rich man, and he will be able to pay Antonio the money back.
Antonio is more than willing to finance Bassanio's trip, but all of his investments and wealth are tied into his ships, which are out at sea. In order to help Bassanio, Antonio gets a loan of three thousand ducats from the Jewish moneylender Shylock on the condition that if he forfeits on the loan, Shylock can remove a pound of Antonio's flesh.