For many reasons.
Research by Helen Prosser and Tom Walley asked GPs why they saw pharmaceutical reps. GPs described reps as a convenient source of drug information, especially for timely information about new drugs. Independent sources of information about drugs were often seen as slower to provide information. GPs also appreciated being able to get quick, in-person answers to questions.
Some GPs enjoyed the social interaction with reps and the ‘time out’ from an otherwise busy day that a rep visit offered. Some GPs found gifts, sponsorship or lunch from pharmaceutical companies important, or simply enjoyed ‘perks’, even when there was an implied reciprocal obligation to prescribe a company’s drug.
Some GPs saw reps partly because it was the normal thing to do amongst their peers in their practice. Some doctors personally felt awkward about seeing reps, but the ‘peer pressure’ from other GPs’ acceptance of reps seemed to lead them to conform. One of the main purposes of the No Advertising Please campaign is to normalise the concept of not seeing reps, even if one’s colleagues see them regularly. (Prosser)
Some doctors also see pharmaceutical reps because they believe that a supply of free drug samples (starter packs) benefits their patients. This belief is contentious. See ‘Will patients miss out?’ [Internal link]