Tree, I'm not sure where you are getting this information, but we have had several planned and unplanned instances over the years where all communications are switches from datacenter to datacenter. The biggest unplanned instance I can remember was the Hurricane in the late 90s. Mahwah was down for a week because the telecoms were under water. Any application that is critical to the operation resides in both datacenters....and the data is replicated between the datacenters and offsite storage facilities....not sure of the frequency.
Same thing goes for the airline. If we lose "operational control" which could mean loss of phones and/or data connectivity, the FAA can ground all of our planes around the world...immediately. Around here, the disaster recovery plan is called ABC. We can run the airline out of another location, and it's tested yearly in both planned and unplanned exercises.
Facilities are different. All costs are absorbed locally, so some centers pay for dual routers, diverse paths, UPSs, etc, while others don't want to pay for any of that.