Because competition and expectations rush to fill the available opportunity, which eliminates the differential between that industry and others.

Pharmaceutical companies in particular are highly dependent on the profit margins from their latest wonder-drug. If they don't have a deep research pipeline, they stand in serious danger of winding up in a credit crunch. Also if the healthcare system ever faced up to and dealt with its addiction to the latest and greatest drugs (particularly when there are workable generics), then that entire sector would face significant pressure on their business model.

How am I aware of this? Because I actually worked in one for a bit (consultant to a cost-cutting unit). How about you?

Cheers,
Ben