Creationists like to use this as a “gotcha” argument against so-called “evolutionists”, but in reality the question is fallacious at best, and utterly meaningless at worst. First thing’s first, one can easily reverse this question and ask the creationist if they were “there” at the creation of the earth by God, of course they’ll say “God was there”, ignoring the fact that the only “evidence” they got is a book shown to be completely unreliable about damn near everything back to front, their God couldn’t even get his damn creation story straight, so why should we view the Bible as reliable since the whole damn thing is riddles with contradictions?
The second problem with it is that it assumes that eyewitness testimony is the most reliable form of evidence, when we know for certain that the case is the exact opposite of what they believe. Eyewitness testimony is the least reliable form of evidence, not only because memory is prone to error, but because the supposed “eyewitness” may have biases or illicit motivations that may cause them to commit perjury and lie about the events they witnessed.
The third problem is that it, probably unwittingly, disses the forensic sciences. Why do all that work investigating a crime scene when the eyewitness said some random person do it, and if eyewitness testimony is the most reliable form of evidence, they must be right, right? Science is nothing more than forensics, we uncover what happened in the past based off of the evidence left behind, there’s always a trail to follow, and that trail doesn’t go away, no matter what the irrational, paranoid, bigoted liar for Jesus wants you to believe.