Yeah, I think it's (at least partly) that you can't believably work backwards from "Guilty" to "Not Guilty". So before you know exactly what the prosecution will do, you proceed as if they've got nothing on you, because it looks the most innocent... and just in case it's actually true. Then, in the event that they have evidence there's no real way to deny, you start working toward a plea bargain or whatever.

(Also: these are my ignorant assclown opinions based mostly on TV shows... NOT A LEGAL EXPERT.)