Because as I said, most humans would never even think of the doomsday argument. So the argument can't apply to them. In order to get the mathematical guarantee that 90% of people who use the argument will be correct, you need to restrict your reference class only to people familiar with the argument.
More generally, the copernican principle is that there is nothing particularly special about this exact moment in time. But we know there is something special. The modern world is very different than the ancient world. The probability of these ideas occurring to an ancient person, are very different than to a modern person. And so any anthropic reasoning should adjust for that probability.
"The probability of these ideas occurring to an ancient person..."
In the ancient world it was very common to predict the imminent end of the world.
And in my own case, before ever having heard of the Doomsday argument, the argument occurred to me exactly in the context of thinking about the possible end of the world.
So it doesn't seem particularly unlikely to occur to an ancient person.
If it's worth saying, but not worth its own post, then it goes here.
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