Because people’s utility curve is not linear with $. Because people, for whatever reason, are less likely to want to risk giving something up than to risk something they never had. So i think a significant fraction of the question boils down to the extent to which a person feels they already “have” the thousand dollars. And the utility of that $1000 to them. If a thousand is trivial and a million is life changing, you are likely a one-boxer. If the thousand is life changing, a person is much less likely to risk it no matter how persuasive the computer is.
And because all my training and experience tells me to be distrustful of novel situations that look too good to be true. I’m still me in this hypothetical. On a fundamental level, i don’t trust this computer, and wonder if the thousand people who went before me are made up, or confederates in some complicated scam.