Based on the Gates foundation's existing accomplishments on eradicating diseases, of which Malaria and Polio are shining examples.
While we've had Polio vaccines and Malaria pills for years in much of the western world, much of the poorer parts of the world aren't so lucky. So instead of duplicating something that already exists and works well, partner with them and get it done faster together.
It's possible for them to work together through open research and collaborative efforts, if their work ends up crossing paths.
But it's not clear there's any reason or benefit for them to be under the same central organization.
Plus they will be greatly incentived when working under their own label 'Chan Zuckerberg Institute' to make their own lasting impact on the world. Just pumping more cash into Bill Gates existing project isn't a very exciting driver. And motivation/morale is important with any big project.
Hey, Bill Gates is pretty excited. Maybe you could tweet him and ask him what you don't get?
“Thank you mark, it’s an honour to join you all after this incredible milestone. The Chan Zuckerberg initiative is already doing some very promising work in terms of improving education for all students (and so) that is a really thrilling thing.
And it is amazing that today they are taking on another bold challenge, this idea of curing and preventing all diseases by the end of the century. That’s very bold very ambitious, and I can’t think of a better partnership to take it on,” Bill Gates said.
Based on what? The Bill & Melinda Gates Foundation has plenty of money. Better for the Zuckerbergs to focus on other areas.