> And 5/21 might be the birthday of your first child, or your wedding day, or whatever?

How sure are you that you'd remember all that scheme for 20 years? How about 50 years? Some documents may be relevant for a very long time. What about if you need more than one key? What about if you need to give access to one document to specific set of persons?

Once you consider all the scenarios that can happen through a lifetime, you start to understand why managing all those complexities correctly is not trivial. And that's why people pay third parties to do it for them. It's not because encrypting a bag of bytes is hard. It's because of all the things that surround it.

Yeah, it's one of those things that you'll forget in N years. That's exactly what prompted "where do you keep secret.txt" question.