None of those OSS relational DBs offer Amazon lock-in the way DynamoDB does - it's more reliable income if someone uses it, but it also takes more convincing for people to use it. What enterprise would use it if Amazon themselves don't?.
Amazon owns Dynamo DB, and make it available via AWS. If customers use it for non-trivial applications, they have some degree of lock-in, because they can't just migrate to another (self-)hosted instance of e.g. Postgres, MySQL, Oracle, Mongo, what have you.
So Dynamo is an opportunity for Amazon to generate lock-in through their own proprietary software.
But big and likely even medium size businesses are less likely (compared to tiny companies that barely go above the free threshold) to use a new technology without any big well known users, or publicly documented use-cases etc.
One big way companies can provide some confidence to potential customers about their technology is by dogfooding: they use the thing they're trying to "sell" (regardless of whether it's a licence, a service, whatever).