Aren't multi-az deployments more expensive? That would be a valid reason not to check this checkbox, if your business can survive a bit of downtime here and there.

Most of that expense is just the cost of a hot failover, but there is some additional cost around inter-AZ data transfer. If someone is not checking the boxes for cost reasons, I would be surprised if they had failovers in the same AZ. It seems more likely they just don't have failovers.

A checkbox that might 3-4x the cost.

multi az brings multi complexity in terms of data duplication, consistency, if your app wasnt designed to handle those kind of scenarios and experience high users loads then you are in for a lot of problems.

designing for those scenarios increase complexity; cost; architecture style and most of the time it will bring you in microservices territory where most of the companies lack experience and just are following best practices in a field where engineers are expensive and few

RDS just has a button for multi-AZ primaries. No complexity or microservices.

ok lets say you have a master in one AZ and it dies. what happens?

Automatic failover within 60 seconds.

ok then what happened to your transactions that were in the middle of a process? the one that were commit only on one side?