It's a holdover from who knows how long ago - Windows 3.0 days at least. And you're right, it never takes that long. My guess is the message comes as default with the installer builder(*), and few people bother to change it, since packaging an installer is generally tedious and painful, and you want to minimize the damage. Or maybe it's even hard-coded?
Also, a professional setup builder comes with localized resources for many languages. So you'll be unlikely to go and change the default messages. If you did, you'd have to have them localized from scratch, so why bother.
But you know what - more and more installers now are stubs that download components from their home websites. Then the message is relevant and true again. We've come a full circle.
(* InstallShield? are they still in action? I mostly see InnoSetup-based installers these days.)