It is universal practice within cloud service providers to span redundancies across "fault domains" - basically things which could make failures correlated, like being in the same machine/power strip/datacenter/geo region. If you assume your fault domain analysis is good, then your failures should be independent. Many global outages are the result of a previously-unidentified fault domain, like the Azure certificate issues. Of course past a certain point it becomes unimportant - who cares about your data if an asteroid takes out every datacenter on Earth at once?