The story of Noach and the flood raises two obvious questions for most people.
First, what could the human race have done that was so terrible that it had to be wiped out. Second, why would an omnipotent G-d bother with the whole business of constructing an ark. Why not simply send Noach and all the animals to the top of the highest mountain. Or simply prevent the water from entering the area where he already was.
The answers to these questions are interrelated.
G-d tells Noach that the end of all flesh comes because the earth is filled with robbery. Robbery seems like a peculiar reason. Certainly it's a sin, but why is humanity doomed because of it?
To understand that we have to look at the nature of robbery. Mass theft on the scale related by the Sages had eliminated the very notion. Theft was so widespread that property had literally become theft. No one could have anything that was legitimately his because everything had been stolen at some point and theft had become legitimized.
Let us consider the nature of property in the religious sense. Everything that a person has is obtained in partnership with G-d, as Chava said of her son. The purpose of a sacrifice is not to provide G-d with a burned animal. He does not actually need the animal as the prophets tell us. It's to pay tribute to the partnership. By bringing a sacrifice, we are saying that our accomplishments are due to G-d.
This is one of the most fundamental ways of worshiping G-d.
One of the things that G-d despises most is a sacrifice brought with stolen money since we are making Him a partner in our crime. And He cannot have any part in stolen property. Widespread theft made it impossible for people to recognize or worship G-d. And made it impossible for G-d to play a role in their lives. All life had become perverted since it could no longer connect what it had to G-d.
The generation of the flood had made it impossible for G-d to have a place in the world.
By commanding Noach to build the ark from scratch, a pure vessel was created to perpetuate life on earth. The ark was not merely a vehicle to save Noach and his family and the animals, it was the reason why they were saved.
By building the ark Noach showed why he was a righteous man. The 120 years spent building the ark brought something honest into a completely dishonest world.
The ark was built from scratch. It incorporated no stolen property. It was done in true partnership with G-d and served as a symbol of everything that the world should have been and no longer was.
Noach's ark was a small world in miniature as it should have been. It was a miniature earth. A model for the relationship between humans and animals and between man and G-d. In partnership with G-d, man had made a world, and G-d had responded by making that world able to bear life.
The generation of the flood had corrupted the world by corrupting the nature of property thereby excluding G-d from the world. The ark had not only brought Noach through the flood, it also brought back Godliness into the world.
Noach invested physical property with moral value. He made the ark more than wood, he made it into a temple. Through his honest partnership with G-d, he showed that physical objects could become sacred again in a world too corrupt for sacredness to exist.
The ark had to be built for the same reason that the world had to be destroyed. Like the Ark of the Covenant, Noach's ark was not only a physical symbol, it was a tangible connection between man and G-d.
Had Noach not built it, that connection would not exist. G-d can make anything, but it's up to man to labor and make a connection with G-d.