In addition to that, there’s the problem of salience. On the smallest scale, the differing meaningfulness of a moment means that certain people attribute more weight to one particular moment than another, even if both have equal importance. On the grander scale, it’s very hard to see whose words or whose actions actually precipitated what events. There are many things that we don’t know, and will never know, that played an important part in the story of humanity.
I suppose we can see it like this: “History” is not synonymous with “the past”. What we call history is nothing more or nothing less than the reconstruction of the past, and the historian is a re-creator whose task it is to build scale models. But the historian, unlike plane or architecture enthusiasts, cannot work from a full size model. He does not have access to exhaustive and comprehensive blueprints. Instead, he has to piece together his scale model from leftover fragments, like trying to build a replica of a plane using nothing more than the remnants of a crash, whilst having no idea what type of plane it was that snuffed it.