Because this is literally exactly how Absurd happened 2 years ago. We never heard Axl soundchecking it, literally no one saw it coming, they played it at the Boston show on a Tuesday (also like the 2nd or 3rd show of the tour if I remember correct), then in NJ 2 days later they said it would be released at midnight. So that is the pattern that makes it a reasonable guess that it appears tonight.