You're naive if you think this doesn't happen more often than not. Some shops are very good about preventing this, most enterprises are not. Drop a bunch of USB sticks in the parking lot or other common areas, someone else ends up doing the work for you.

I do incident response for a living. Disgruntled employees/former employees do sabotage their companies' networks, but it is incredibly rare. Not because most organizations are well-defended against such attacks; its because most people aren't willing to destroy their lives simply because they were laid off. Especially in a rapidly growing industry where a talented person can generally find a better job than the one they had in a week or two.