With nuclear, the natural state of fissile material is inert, under critical density. It's too spread out to lead to a spontaneous planet scale detonation. You have the work hard to amass even tiny amounts of fissile material for experimentation. You can't blow up Earth even if you want it, there isn't enough fissile material on Earth to do so.

With bio, the fissile material is humanity itself and is above critical density. Once the virus escapes your lab, you have a planet scale pandemic detonation on your hands. Afterwards, all you can do is to try to spread out the fissile material [aka humanity] to keep it under critical density. See 'social distancing' and 'stay at home orders'. Which doesn't work long term, note the economic crisis that's just starting, arguably worse than the Great Depression. What positive rewards are you expecting from this? A new source of energy ain't it.

Imagine a Manhattan project where, if a handful of stray neutrons escape Oak Ridge, the entire Eastern US goes up in flames.

Edit: How about: 'learn how to contain' nuclear detonations by partially detonating planet-scale doomsday devices?