Even when they do, there is a problem. They specialize in DB code. AWS specializes in infra... and can just take the DB code as well. That is a pretty huge competitive advantage (while the db company has to develop that second specialty). Even worse, there's the further monopoly advantage that companies can just role it into their AWS bill instead of doing another vendor evaluation. The fact that Amazon has almost all the competitive advantages for offering someone else's code it what seems unfair.

(the one they don't have is the ability to offer super specialized support and modifications for their customers, which is where a lot of the database companies have made their money - on consulting)