While there certainly are concert pianists who are excellent teachers, they are usually not a good fit for most students. Here I've gathered some reasons from my own experience studying under teachers with an active performance schedule, and parents of students who have studied with concert pianists before transferring to me.
1. A great performer doesn't equal a great teacher.
2. A concert pianist's commitment and priorities are to their performance career and rightly so; not their student's pianistic developments.
Once, when I was backstage at one of my previous teacher's performances, she apologized to me. She apologized and said I was one of her biggest regrets as a teacher because I was her most talented student at the time and she felt that her absence had driven me away from the instrument--this was my violin teacher.
3. It is very likely that many things about playing piano come naturally to a concert pianist; things they never had to learn to do or be taught to do.
4. Unrealistic expectations for students in terms of speed of learning and amount of practice is another possible cause of rift between a concert pianist and the student.
5. A concert pianist most likely started learning piano at a very young age.
"Just do it" might be a great slogan for Nike, but it is horrible for teaching, and many things I do now while playing piano really has become that, I "just do it". My studies in piano pedagogy in graduate school forced me to return to how basic musical concepts and expressions should be taught, things that I do naturally now and take for granted.