They don't do that because a) they won't work on it all weekend since I doubt the bigwigs would pay them the overtime, and b) if they give a time like 12-24 hours, the first person that has to wait 24 hours and 10 minutes comes here and throws a hissy fit about them not keeping thier promises.
Heck they have people that complain when the do a hotfix or patch and say 'we estimate it'll be down 2 hours' people raise cain when it's down 2 hours and 5 minutes.