For me, brewing in a small craft brewery is the most (or at least one of the most) fulfilling careers I can think of.
It is a near perfect blend of science, craft and art. On some level, if you don't understand the underlying science behind making quality beer, you will be limited in your development and ability to create fantastic beers day after day. With the history of brewing stretching over 7,000 years, and new research constantly coming out, you could learn something new about beer everyday for the rest of your life. Brewing beer day in and out becomes very craft-like, as you work to perfect the details of creating the same beers using a continually shifting set ingredients and conditions. Beer is a live agricultural foodstuff, there are things you can control, and things you can't. A good brewer tries to learn the difference, and make the best possible beer every time. Then the art. An accomplished brewer blends the science, and craft, and elevates them into an art. Creating a creature of beauty, that can bring wonder and joy into the lives of all it touches. Sometimes the magic is there, sometimes it is not. The brewer for whom it is more often there, becomes the artist. In a nutshell, that is why I am a brewer. Its just what I do.
As for brewers leaving the industry, that's a big question with as many answers as brewers. Craft brewing can be intensely physical, with long hours, and often inadequate pay. Some brewers just wear out so to speak. The aching backs, the low pay, management that often has very different priorities than your average brewer. Management, especially in brewpubs, sometimes doesn't understand exactly what a good brewer does, needs, and can offer, and this can lead to disagreements and brewer dissatisfaction. (This can of course go both ways with many brewers not completely grasping the bigger picture of restaurant operations, and where they fit in.) Hubris or the primadona complex, often seen in brewers, rock stars, and celebrities of all walks, has also reared it's ugly head to the detriment of more than one career.
In many fields the easiest way to quickly raise your salary is to move from job to job, and you do see many brewers (and even more chefs) doing this as they look for higher pay, and greater job satisfaction / management relations.
I hope I didn't go on too long, or offend any delicate sensibilities,