AWS makes it pretty easy to operate in multiple AZs within a region (each AZ is considered a separate datacenter but in real life each AZ is multiple datacenters that are really close to each other).

That being said, there is still an added cost and complexity to operate in multiple AZs, because you have to synchronize data across the AZs. Also you have to have enough reserved instances to move into when you lose an AZ, because if you're running lean and each zone is serving 33% of your traffic, suddenly the two that are left need to serve 50% each.

The bigger companies with overhead reservations will get all the instances before you can launch any on demand during an AZ failure.