Tennis is a difficult sport to master and it takes a great deal to break out of the beginner catagory . Peoples skills are based on their coordination (hand/eye), spatial skills (ability to sense lengths and widths), reflexes and physical movement
Like golf, tennis takes muscle memory and your ability to get into a "form" or repeatable body movement. Muscle memory gives players the ability to hit with accuracy and consistency. To achieve this it takes numerous amounts of repetitive strokes to gain any consistency with the ability to hit the ball.