I don't know who said it first, and to me it sadly seems like to suggest a rather negative intention when spoken, but the phrase "those who can't do, teach" is quite common and as someone who genuinely enjoys teaching and sharing what I know, see it as a phrase that can actually lead to some valuable insight.

I think the original intention of the phrase was to suggest that teachers are those incapable of doing - for who in their right mind would rather coach, say, Sydney Crosby instead of being Sydney Crosby?  The phrase is also often believed to imply the opposite - that those who can do would not want to teach.

Doing is Generally More Lucrative than Teaching

It seems that most of the time we see those who can "do", doing it.  It seems quite unlikely that an individual who is the most talented at the world in something would rather teach and give up the title of "best in the world".  Besides the pride involved, one key incentive to doing is that it is significantly more lucrative than teaching.

While Roger Federer could probably be the highest paid tennis coach in the world, it's unlikely anyone would be willing to pay him $36,000,000 a year to coach instead of play (the amount Roger Federer made from his tennis winnings and endorsements in 2009).  It's just way more financially lucrative for those who are the "best" at doing to do instead of teach.

But Just because you can Do, doesn't mean You Can Teach (Well)

It takes more than just being good and/or knowledgeable at something to be a good teacher.  In order to be a good teacher, you need to also be good at communicating your ideas, have patience, have a genuine interest in the success of your students, adapt to different student learning styles, etc.

I'm sure we've all had professors who are considered brilliant in their fields but are unable to convey their understanding in a way that the class could easily understand.

Teaching is an art and a skill.  It's challenging and rewarding.

Someone who is of more moderate skill level at doing but better at communicating their ideas is often a much better choice for teaching than someone who is greatat doing but has difficulty with conveying their ideas clearly.

So even for someone amazing at doing, they might continue to do simply because they know they wouldn't be a good teacher.  Or even if they could be a good teacher (if they put the time into developing those skills), they might genuinely just not be interested in teaching.

Of course, Some people Love to Teach

Nonetheless, you are always going to find people who are great at doing, but still want to teach anyways.  There are some people that just love both doing and teaching, and are great at both.  It's always a nice sight when this happens.

Teaching is Often Lower Variance and Less Stressful

Quite often you will see people who are really good at doing but choose to teach instead.  One major reason for this is simply that teaching has (relatively) guaranteed financial rewards but doing does not.

For example, Roger Federer only gets paid if he wins but his coach (when he does have one) gets paid no matter what (assuming the coach is good.  A smart player is not going to fire a good coach unless he thinks his loses are because of the coach).

A great tennis player, for example, can go through phases where he is injured, chokes under pressure, gets very unlucky, etc which affects his pay.  Throughout all this, the player's coach is usually paid consistently.  While it's true that the coach will likely also be affected by the player's results, the consequences are relatively less severe.

As a result, when doing becomes too overwhelming, teaching is a viable and rewarding option.

There are of course exceptions here and there, but I hope this article provides some insight on the doer vs. teacher relationship and what factors affect the route individuals take in their careers.