Because you have to get a magistrate or judge to approve the issuing of the search warrant. A magistrate or judge acting properly will not approve a vague fishing expedition. The police need to be specific as to what the crime is they have probable cause to believe occurred, and what items connected to the crime they believe may be at the premises to be searched.
Once the warrant is approved and the search is underway, however, the police are allowed to lawfully seize any other contraband they may come across, including the cocaine in my example.
If the police lied to the judge about the basis for the warrant (for example, lied about the basis for having probable cause that a crime occurred), and did not proceed in good faith, then evidence seized can be suppressed, whether the evidence pertains to the sexual assault they are investigating or to another crime like cocaine possession, on the theory that the police did not have a lawful right to be in the premises conducting the search in the first place.
And, by the way, note that there was no coke or pot or anything else returned out of the house.