Drug development is incredibly costly. It's also something of a long term gamble. It can take 10-20 years to develop a new drug. A lot of that time is spent testing safety and waiting for approval. As a drug company, you need to have several new drugs in your pipeline at any given point, or you'll be seriously fucked if one (or more) of your products don't pan out. It's basically a high-risk-high-reward type of gambling situation.

Imagine having thousands of people on payroll, some working on drug design, some designing experiments to test the drugs, some doing the legal paperwork to get drugs approved etc, and suddenly the patent for your best selling drug expires and your latest drug doesn't work. You're pretty much boned.