In most cases, you learn more with a great teacher than you do by yourself, and teaching is a different skill than performing. Even Tom Brady has a quarterbacks coach, who couldn't throw the ball like he can, but can still offer constructive guidance.

This is especially true for complete beginners. If I sat down in front of a piano and practiced an hour every day for six months, I would probably not do very well, since I wouldn't know where to start, I wouldn't know good technique, and I'd probably learn a lot of bad habits. If I took just a few weeks of lessons and then practiced an hour a day for six months, I'd make a lot more progress since I'd have a better idea of how to learn piano.