Not sure why there is a lot of negativity here. It doesn't seem that ridiculous of a goal. I think by 2100 we will have extremely powerful AI that will make fighting diseases extremely easy compared to methods available today. To be honest, it seems like an achievable goal. I wish Zuckerberg and Chan the best in this.
Because there is a widespread dislike of Zuckerberg on HN and most other similar discussion forums. Happens in every thread about him. He's an uber billionaire that got rich serving ads on a social network that couldn't be any more mainstream at this point. Facebook is viewed mostly as a privacy invading corporate monster now by the crowd that dislikes Zuckerberg. He plans to donate all of his wealth and what you mostly saw on here in discussion was venom and conspiracy theories about how it was nothing but a tax dodge (which only betrayed the vast ignorance on tax laws).