Why all trees have cylindrical stems?
1)
You know most bridges have vertical cylindrical pillars. 

This is probably because cylinder is among the strongest 3D structures to support anything.
So, cylindrical trunk provide best support for tree crown.

2) Most accurate
It is natural for any cell to grow equally in all directions. This, leads cells to expand in all directions equally in a circular girth.

3)
Resistance 
Cylindrical girth provides least resistance to air. So, tree naturally takes this shape to prevent damage by air strikes.

4) Read
 https://m.reddit.com/r/askscience/comments/1973mh/

for more info.

5) http://www.stevespanglerscience.com/lab/experiments/balancing-act-cylinder-strength/

for strength of Cylinders.