I think there is a good reason. Who wants to spend the time setting up and running a server? I happen to run my own, but it is definitely not something I would recommend to my friends and family.
Maybe someone will come along and create a super easy to install and low maintenance server platform, but if everyone uses that then it is still "centralized" in the sense that everyone is running homogenous setups. I think the key is that everything should be based on open protocols and formats. Email and the web are already there. Tent.io might be a good place to start for the social media stuff. That seems far more important than geographical dispersion of physical servers.