A multi-az deployment is a checkbox in most AWS services, e.g. ASGs, RDS, load balancers, etc. Someone didn't check that box because they didn't know about it, there isn't much complexity in it.
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