A general defense is that nature is a far better incubator and gain-of-function laboratory that we could ever hope to be. A few billion birds have caught bird flu in the past decade, so we're talking about trillions and trillions of viruses constantly undergoing mutation in close proximity with farm mammals and humans all over the world... If we know the genetics of what turns a bird flu into one transmissible to humans, it's probably better to "get in front of it" rather than just awaiting that mutation to happen somewhere in nature for the first time.