The issue that I see is that DNA isn’t necessarily unique. For example, identical twins and species that were born asexually can have identical DNA. I don’t really know any biology, so maybe there are other considerations too?

Also, hash collisions would happen randomly sometimes, so even if every individual had unique DNA, you couldn’t use them as a unique identifier since there would sometimes be collisions.

Hope that helps.

Edit: I was wrong about hash collisions. Also, removed last paragraph because I was repeating myself.