It's a lot easier and faster to destroy than to defend. To defend, you need to know what you're defending against, develop the defense, and then roll it out, all reactively post facto.

If a computer has the ability to quickly make millions of novel viruses, what antidotes are you hoping for to be rolled out, and after how many people have been infected?

Also, if you follow the nuke analogy that's been popular in these comments, no country can currently defend against a large-scale nuclear attack--only respond in kind, which is little comfort to those in any of the blast radii.

300m dead humans later, we’ve nearly eradicated it, or perhaps found a way to live with it

It’s a very asymmetrical game. A virus is a special arrangement of a few thousand atoms, an antidote is a global effort and strained economy