The difference is between tacking aircraft in a training environment and the reality, that you are responsible for keeping these sardine tins in the air and separated.
Some people along the way just decide it isn't the job for them, it wasn't what they thought, their priorities change etc.
Some people just can't visualise their airspace as four dimensions, the forth being time, so it looks easy flat but when you have FL and objects moving etc it becomes alot different.