Shouting into the Abyss - Why do some "easy" observations sometimes go unidentified?

As a researcher using iNaturalist data, it’s useful to be able to extract all the observations that I have personally confirmed the ID of. Particularly for groups where I need to maintain a higher standard of confidence than “research grade”. In some cases I’ll leave a coarser ID (ie., a genus) and an explanation as to why the species can’t be verified from the data presented. But for common species that can be a lot more work than just selecting out the observations that do have adequate images.