For most of the time courts have existed, eyewitness testimony was the only thing available in criminal matters. From a modern non-legal perspective it does seem little strange that it's so heavily weighted when we have photography and video recording and scientifically based evidence but the idea is that the jury and/or judge is supposed to work out whether it's credible and not just assume anything said is true.

Unfortunately research suggests that any eyewitness is by definition an unreliable narrator because memory is not an objective record that can be perfectly replayed at will but either we say that any conviction prior to the invention and widespread use of, say, photography, was actually unsafe and leave many modern victims who didn't happen to be recording the event without justice, or we accept the issues and attempt to mitigate them as best as possible. Right now the law says the best way to do that is a large number of people examining the testimony and deciding amongst themselves whether to believe it.