If you wanted to deploy something similar, like Cassandra across AZs, or even regions you're welcome to do that. But now you're on the hook for the availability of the system. Are you going to get higher availability running your own Cassandra implementation than the DynamoDB team? Maybe. DynamoDB had a pretty big outage in 2015 I think. But that's a lot more work than just using DynamoDB IMO.
this is both more and less true than you might think. for most regional endpoints teams leverage load balancers that are scoped zonally, such that ip0 will point at instances in zone a, ip1 will point at instances in zone b, and so on. Similarly, teams who operate "regional" endpoints will generally deploy "zonal" environments, such that in the event of a bad code deploy they can fail away that zone for customers.
that being said, these mitigations still don't stop regional poison pills or otherwise from infecting other AZs unless the service is architected to zonally internally.
[1] https://www.reuters.com/article/us-france-ovh-fire-idUSKBN2B...
[0]: https://aws.amazon.com/blogs/aws/in-the-works-aws-canada-wes...