Because even the most-thorough search might miss something, basically.
The example (from my games) I have in mind is this: The party had killed the Evil Wizard and his Mooks, and they searched his quarters. The party found a lot of stuff, including money and items and information. They had a chance, even taking as much time as they wanted, of not finding the secret door that led to the chamber where his clone was stored.
They failed on the Int(Investigation) check/s to realize that the volumes of notes they found were A) fragmentary and B) on paper that was too close to the same age to match up with the dates on the notes, so it never occurred to them that these notes, and the treasure they found, were there with the intent to convince someone that they'd found what there was to find.