If you are Demis Hassabis and want to lead this new initiative:
- Why would you want to report to the CEO of another subsidiary instead of spinning-off another entity within the Alphabet conglomerate?
- You can let the DeepMind team focus in their original areas of expertise.
- You will have a new (and $$$$) budget to hire computational biology PhDs
- With the new entity, you can fail a lot and not make Verily and Calico look bad (inside and out XYZ)
If you are Verily and/or Calico:
- You don't want a newcomer bring the exciting new projects and ruin your projects, budgets, initiatives
- [very speculative] You don't like Hassabis' progress (or is envy of)
- You think this new project will fail and don't this under your umbrella
Calico does basic life science research and publishes it, https://calicolabs.com/publications and also has private partnerships. They are pretty secretive so it's hard to know but most of the research is about using mouse models to understand fundamental details of aging biology, the long game being to make Larry Page live forever.