You have 3 basic people:
1) The Squatter: The person who gets a position with little competition like in specialty drugs. They have very little doctors to see and spend most of their time on the golf course making big bucks. They will never leave unless they get bored of making a lot of money and not doing anything or policy changes force them to actually do some work.
2) The first timers (Churn and Burn) - first job in Pharma, they think that it's going to be cake but they end up sitting in waiting rooms for hours, spending a lot of time by themselves. They get bored and move on, either to new positions in the company, other companys, or outside the industry.
3) Hoppers (Incestuous) - these people just work the system. They get into Pharma then hop around companies, each time increasing their pay. Typically don't last more than a year in one company.
Really, the latter 2 are the reasons you see so many job openings.