That's what's called the learning curve. When you are first introduced to a skill, you pick up the basics very quickly. Note how quickly people learn to drive a car. However, it then becomes harder to improve, and learning becomes more specialised. For those interested, the learning curve for motor skills is much sharper than that for intellectual ones.
Motor skills are often harder to increase once competency is attained.
Not a bad idea, but there would still be some actual improvement as opposed to reshuffling. And even though ability decreases without practice, it is quickly regained. I'm not sure why they made this change, but it was probably because CoC characters tend to have a hard time lasting long enough to become truly good at anything anyway. There is no limit on the number of skills that can increase. You can also get points for training.