I see you've asked this question so many times, looks like you are thirsting for knowledge. Well, the reason is that though Ram was God Vishnu incarnate, his role was to live like a human. Hence he is called "Purushottam". And like a true human, he used whatever resources was available. He had the help of his friends - the Vanars - and he didn't have the tools to build a boat or a ship. Since stones were abundantly available, the vanar sena built a bridge.
