A traffic stop can be initiated for almost anything...burned out taillight, failure to signal a turn, swerving in traffic, known driver without a license. When asked if they can search the vehicle, many people say yes. Also, drug paraph. is often visible just by looking in the window, the cops then have probable cause to search the vehicle.
A search warrant on a house becomes much more difficult to obtain. Most people do not have drug paraph. sitting in their house window, evidence must be submitted to a judge to substantiate a search warrant.