Simply put, the number of arrests an officer makes is a major factor used to determine his job performance. Police officers know that the easiest way to make arrests is to find people in possession of illegal drugs, so they want to search everyone they can find.
Statistically, for every ten people an officer searches at random, he will make one drug arrest. This means that nine innocent people will likely endure the inconvenience and humiliation of a police search so that one law-breaker can be arrested. In some officers' minds, the nine searches that turned up nothing are easily justified -- especially if those people willingly consented to warrantless search requests.