Perhaps the distinction is just because something isn't a "core competency" does not mean it is not critical. And just because it isn't a "core competency" doesn't mean you can afford to be ignorant on the topic.
- the database must be secured against unauthorized or improper use
- the database must be accessed efficiently
- the database must have some degree of resilience
- the database must be recoverable
- the database must be available
Each business will have different weightings for these properties -- does a dating website care more about database security than a bank does? -- which will lead to different decisions about who should own and manage the DB.
But a business is, absolutely, a group of people who are doing things together. If you can't identify, authenticate, and authorize those people, you don't have a business. And if the database that stores that information isn't among your highest priorities, I think that business is rather misguided.
Sure, this is hyperbolic, but it does illustrate that not everything is a "core competency". There are always tradeoffs and decisions when it comes to software.
In my opinion, authentication and authorization are likely not core competencies for 99% of businesses. They are simply capabilities that just need to work and the business doesn't need to own them. This is the same with the database. Using a tool like FusionAuth, Keycloak, or SuperTokens will cover all of their needs.