The animals are already dead

Yes. But veganism is not about saving the dead animals in the supermarket, it is about reducing the demand for animal products to prevent further animals from being bred and killed. This is the basic Supply and Demand theory from economics. If we demand more animal products, the businesses that produce them will supply more, therefore killing more animals. We have the power to vote with our money, and every time we pay for an animal product we’re indicating we want more of it.