IMHO, there have been quite a few good posts regarding the question asked at the beginning of this thread. I would like to just give a very brief opinion from my perspective, as a golf pro who gave his first lesson 44 years ago, and have worked with beginners through touring pros.
At the outset, I ask many questions of a student a few days prior to our initial evaluation lesson, or at the beginning of the lesson if they were scheduled very recently. These questions are in no particular order but I would like their answers to as many as possible.
1. How long playing golf?
2. Any physical limitations?
3. Avg. score or USGA handicap?
4. Biggest frustrations playing golf?
5. What are their perceived strengths/weaknesses in golf?
6. Previous lessons?
7. Any current goals in golf?
8. How often do they play/practice?
9. Other sports playing/played?
10. How would they rate their eye/hand coordination?
11. Do they believe they are a patient person?
12. How would they rate their work ethic?
13. Do they play for pleasure, business, or to be competitive?
14. Do they believe they are a competitive person?
15. Do they find practice to be helpful/not helpful/boring/fun?
16. Are they willing to work to build a better game/swing?
17. What are their strengths/weaknesses in golf?
18. Do they have subscriptions to golf publications?
19. Do they watch the Golf Channel or maybe YouTube golf instructional programs?
20. Do they know if they are right or left eye dominant? (I'll test to find out if they aren't sure)
21. Do they want to understand the physics and geometry of the golf swing or would they rather just leave all of that to me without them having to think of such stuff?
Based upon their answers to general questions like these, I may or may not ask more questions. It is extremely important for me to get inside their heads as much as possible if I am going to give them the most efficient help and also teach them how to enjoy the process much, much more than they may have ever dreamed they would.
Having said all of that, if I was to list the major reasons why people don't improve at golf, or improve very slowly, the following would be some of the basic reasons:
They practice incorrectly
They don't understand cause and effect related to the golf swing
They don't have clearly defined goals
They are too drunk on watching their ball striking quality to focus on building better fundamentals
They find practice boring
They don't believe they can get much better because they have erected prison walls around what they perceive is their talent level
They listen to too many people giving advice
They fail to have a clearly set goal for each practice session, which leads to confusion and more drunkenness on their ball striking at present
If they fail to plan, they plan to fail is a very accurate statement when it comes to improvement that is consistent and reliable
My company slogan is this; "Building World Class Golf Swings...one swing at a time!"
I can teach any golfer who has fairly good health, to have a world class golf swing. Yes, yes, I know...that sounds awfully arrogant, but once I show them how simple it really is, and show them how fast they can improve if they are willing to trust me, they are usually surprised how fast they can start to get into quality positions in their swing.
The reason why I gave my two cents in this discussion, is because the commonality between improvement at golf or pool, is dealing with human nature. When I show my students how to steadily improve, and that improvement is not left up to the gods, but is in their control, they start to break down those prison walls.
The great thing about all of that though, is that they soon see that many of the mental weaknesses they have related to golf, are the same weaknesses that keep them from more success in life, relationships, business, or maybe their acceptance of themselves, as it relates to self-esteem.
What I love about all of it, and why I am so passionate about my work, is getting to see the epiphanies people get, intermittently as we work together. I also love seeing them experience freedom that they might not have felt since childhood, when they start breaking down those prison walls, which they erected themselves over years or decades.