I find that there are very few things related to programming that don't constitute an interface in some way, so it feels natural to me to think about everything

I find that there are very few things related to programming that don't constitute an interface in some way, so it feels natural to me to think about everything in terms of the interface it presents to users, both explicitly in what you can express via whatever language you're using, and implicitly via conditions that can't be enforced but are required for correctness. And when you have an interface, you really ought to put some thought into its design regardless of how "private" it is. An API is an interface used by a program in some way, and any user-exposed database objects definitely qualify under that definition.