A lot of people will automatically fail over jobs to other AZ's. That often involves spinning up lots more EC2 instances and moving PB's of data. The end result is all capacity on other AZ's gets used up, and networks get full to capacity, and even if those other zones are technically working, practically they aren't really usable.
While there may be more machines provisioned, many orgs run active setups for failover so they aren’t as affected. In terms of data transfer, it should already be there. Where would it come from? Certainly not the dead AZ.
