Storing the information isn't the real challenge, structuring the connections between different pieces of knowledge is what seems to be the very difficult part.
Without the latter, you can have all the data in the world, but no way to make any use or sense of it.
Let's start with your statement: "If it can store and retrieve data, it's a database." One of the notable characteristics of human memory is that it is not reliable. It does NOT reliably retrieve the data that it stored -- and I don't just mean that we forget some things, I mean that many of our memories are factually incorrect.
Human memory is extraordinarily USEFUL, but in order for the term "database" to have a useful meaning, I have to categorize human memory as a form of information storage that is NOT a "database".