A certified flight instructor (CFI) is the person that teaches you how to fly; until you get the hand of the aircraft you are dealing with, especially if it is a taildragger, the flight instructor is also the person that keeps you from killing both and destroying a valuable piece of equipment. It is indeed quite a responsibility to be a CFI.
Some people just love to teach: some become flight instructors after they earn their commercial pilot license and log five hundred hours, and remain as such for the rest of their lives: but for most career-minded pilots, getting certified as flight instructors is just a step on the ladder that leads to the cockpit of an airliner. Most airlines want pilots who have logged at least 1.500 hours flying somewhere else, plus higher ratings.
That is indeed very costly, especially if you consider that just earning a private pilot license takes around fifty hours and costs a small fortune that just a few can afford without a major financial strain. So, logging well over a thousand hours of flight time requires to actually work within the aircraft industry: some do that flying executive aircraft, others become crop dusters, others tow sailplanes and so on, but most become flight instructors because that is the easiest at fastest route: in three years after they earn their CFI wings, they can think of sending their resumes to the almighty airlines.
You can bet your life that a CFI knows about the trade of flying, but the attitude of that person towards student pilots trying to earn their first license or certificate, and in lesser proportion, regarding all those wanting to earn other, higher certificates will be different depending on what the instructor has in mind for his or her career: a person how is just there to log a lot of hours and doesn't mind the relatively low pay received will act in one way, and the individual that does it for purely vocational reasons will teach you in a very different way.
The problem with flight instruction is that there is no career structure related to it, the pay is not as good as in other realms of the aviation industry, and of course, flying with student pilots is somewhat riskier than taking six dozens of tourists in a charter flight from Dortmund to Biarritz. But the fact is that the whole industry would benefit from having a career path laid for people who teach because it is their vocation to do so.