Your target group and the effective user base might not strictly overlap. You can't always anticipate that. Even if your target group is junior devs, it may be hard to accurately imagine their perspective on something you're necessarily more familiar with. And even if you do, investing the time to explain and document may detract from the fun you have actually achieving "milestones" in terms of features. Particularly for many open source projects, the devs' fun is ultimately the driving factor.

It's a start, but not a full solution.