1. Many of the people who are now out of the work force are more unemployable than unemployed. They either don't have the skills employers need (this is more likely the case among older workers), or they don't even have minimal standards of behavior to show up on time, look presentable, and pass a drug test (more likely the case with younger workers).
2. A lot of the long-term unemployed are unwilling or unable -- perhaps for perfectly legitimate reasons like family obligations -- to relocate to another part of the country to take a job that fits their skills.
3. Many of these people who "stopped looking for a job" are simply retired. The total U.S. labor force in the U.S. includes everyone over the age of 16 who is not in school, in the military, or institutionalized, and with Baby Boomers reaching retirement age the U.S. labor force number actually includes a lot of people who have no interest in finding a job right now.
Number 3 is the most important. Tell me if I am wrong, but I think someone noted that once you have been looking for six months, they just drop you out of the measurements (NOT counted as unemployed).