[watwut]

The old saying is designed to make those who cant teach feel good and superior on expense of those who actually spend time to do something useful.

Teaching is a skill. Criticism can help people to learn how to do it better, devaluing everyone who even tries leads to culture where people won't put effort into it even if they could be good. All that because someone needed to feel superior for doing nothing.

[CyberFonic]

I think the saying you are referring to is:

"Those who can, do; Those who can't, teach."