Automatic multi-zone failover to survive az cashes without outages seems to be commonly based on optimism, as people don't effectively drill for it regularly. There are always bugs in it that need ironing out. So people just pay the price in operating costs, keeping replicas running on other AZs, and the resulting sw/distributed system/infra complexity, but but don't actually make the last steps to reap any payoff. It's because single-az outages are rare enough and that still leaves relatively common aws-wide outages you face and your self inflicted footgun outages. And drilling for the az failover can be scary and risky if you haven't done it before and are already in production.

AZ redundancy is like storage backups or backup power, if you don't regularly test it you don't have it.