Thanks for posting a good and extremely valid question. Nursing is a profession and a job. In pretty much every field of work that requires some interaction with humans, you will find negativity such as friction of clashing personalities, ego trips, lack of appreciation and every defect of the human condition. THAT'S WHY GOOD NURSING PROGRAMS INCLUDE MENTAL HEALTH IN THE CURRICULUM. You not just dealing with wild patients!
Anyway to get back to your question regarding nurses who have left the field... it is possible that that type of nursing was something that they were not suited for. I worked in banking, in corporate travel, as a word processor, transcriptionist, as a legal secretary and as a cab driver. Any time you are engaged in a trade that requires either product or service, you will find some unhappy camper on the other end. Want something where you are stress free? Raise goats!
As for me, I don't know if bedside nursing is going to be the last job I will have on this nursing train ride. Not every nursing graduate is cut out to work on one particular station at a hospital or in a long term health care facility. There are other situations where the need is present... discharge planning, evaluating, doctor's offices... not to mention teaching or even research.
Don't let a few unhappy graduates make you crestfallen. If you really want to do that then I would suggest looking at engineers, architects, and accountants who got burned out after graduating. And imagine what pressure might feel like when your bridge collapses or your client gets called to the Federal Trade Commission or the Internal Revenue Service !