When describing bends in piping, joints, and in many other contexts the word "elbow" is used. You go to the hardware store and you see 90 degree elbows. The word elbow is used everywhere. Why not knee? Why elbow?
Well take a look at this picture (knee brace anchor):
There may be a very simple explanation for the "elbow" phenomenon. When was the last time you used your leg to illustrate nonverbally a point you were making--other than while demonstrating a style of kicking?
People, by and large, tend to use hands and arms to gesture. They do so to illustrate, to emphasize, and to show approbation or disapprobation (e.g., the A-OK sign, as with the index finger forming an "O" with the thumb; or with the thumbs-up sign; or the not-so-OK sign, as with "thumbs down" or with the middle-finger salute!). With legs, not so much.
As Pam points out, the knee simile is used, it's just not as commonly used and understood as well as the elbow simile.
While I can't find a source that really defines it, it's been my experience that an elbow is traditionally a twist or bend in a pipe. A knee on the other hand, is typically a bend that connects to a joist or other kind of support structure.
In Pam's example it's a pivoting mount that connects to a support post.