Add to that the fact that it has been so overshadowed by the more recent World War II, which has become a much larger part of pop culture, by virtue of the fact that it has a much clearer enemy that is much easier to portray in films, and it isn’t that surprising that such documentaries are necessary.
World War I is one of those parts of history that tends to get overlooked.
It happened far enough back in history that nobody who was part of it is still alive.