They have the credibility. You don't question if these companies really have the people to keep the stuff running. In many cases the customers also already have a billing relationship with them. Much less friction to just add one new service to the already quite big invoice vs buying from a different company.
Competing with AWS or Google is hard, they would be still making money on the infra even if they gave the managed database out for free.
Rethinkdb is an incredible architecture. If I was in the market for something like rethinkdb/dynamodb, I would completely go with something that is built by the original inventors.
Just like docker for example.