Because many customers have their own expertise in the database system of their own choice. A shop that has hired Oracle DBA's doesn't want to hire Postgres fol

Because many customers have their own expertise in the database system of their own choice. A shop that has hired Oracle DBA's doesn't want to hire Postgres folks. A MySQL based place doesn't want to hire Oracle folks. And they already have built extensive data-management processes around their choice of database systems and have the in-house experience to manage their database system in a wide variety of platforms. If you to sell a packaged, deployable product to the widest market, yet want to minimise support, you leverage the expertise that the customer already possesses. This is probably more of a consideration in the business enterprise space than some simple normal end-consumer SaaS, so I am not saying this approach makes sense everywhere.