Most people become instructors because their only credential in life is that they're good tennis players.
Most good tennis players have little idea of what they're doing. They can tell a student, "Do this" and show them, but that works only with players who are good at imitation.
Most people have an occasional day when they're hitting far better than usual. Those who progress are able to remember that feeling and how to reproduce it. (I never could, so sometimes years would go by before I ever again hit a given shot that well.) But those who do remember and reproduce feelings and build on them -- become good. Such players cannot tell a pupil "This is what it should feel like when you hit the stroke."
Most good tennis players are good one style of play. Back in the 1970s the most common first lesson with a new pro was to have the student learn the coach's grips -- because the grip determines the stance and the swing, and instructor didn't know how to teach any style other than his own.
Most instructors cannot pinpoint the most important thing the pupil is doing wrong and needs to change, and will become obsessed with a triviality (e.g. "Step this way with your feet" -- when actually good players hit the ball with a variety of footwork options depending on the situation).
Most instructors do not know how something about the pupil's physical condition might dictate a style that is suboptimal for other players. For example, after struggling for nearly half a century as a low-level player, I have realized that because my eyesight is ****ty and I lack normal neck rotation, that I would do best using the techniques that were popular in the 1930s. (Players then didn't use nearly as much body rotation as players today; the fast, patchy grass courts limited the time they had to set up -- they couldn't predict until the last moment exactly where the ball would be. So now I minimize the backswing and switched to hitting extremely flat with a near-continental grip to maximize my chances of catching the ball in the racket's sweet spot. No amount of topspin will give enough margin for error to compensate for frequent mis-hits. No amount of racket head speed will replace the power lost by missing the sweet spot. And when you're 77 and, due to limited body rotation, cannot generate racket-head speed, not only can you not afford to divert much of it to the production of spin, but at low speed you even need topspin to keep the ball from sailing out. And I can cover more court if I can hit the ball with the same body orientation that I have when running after it. Even top pros often modify their technique to be somewhat more like this when their ability to prepare is severely stressed, e.g. when returning a first serve. The difference is that, with my ****ty eyesight, an elaborate preparation is _never_ an option for me.)
I don't agree with those who claim that the people who win are necessarily the better players. How good you are is measured by how hard you hit and how much topspin you put on the ball. Brad Gilbert made a career out of beating tennis players who were far better than he was. Likewise, Ken Rosewall not infrequently beat a younger top-of-his-form Rod Laver even though Laver was by far the better player. (Laver hit harder, with more topspin. In fact, Rosewall _never_ put topspin on his backhand.) Top pros such as Kramer and Budge declared that Bobby Riggs was the most underrated player of all time. (Riggs was the #1 pro for a while despite not being very good; i.e., he didn't hit hard.)
So you could say that I've given up on becoming a good tennis player and focusing instead on learning to win (low-level) matches -- by focusing on techniques that will maximize my control over the ball, taking into account the infirmities of age and my life-long overall severely below-average athletic talent.
(P.S. I just noticed that it lists "Professional" by my name. Not even close. I have no idea how that got in there. How do I change it?)