Because if you read a lot of 20th c sci fi, you can easily imagine using the current DNA printing technology and virus databases to design something that would kill in the billions of people (e.g. The White Plague: https://www.goodreads.com/en/book/show/93448.The_White_Plagu... )

And they all seem to have been fans of 20th c sci fi (albeit sometimes with profoundly poor comprehension).