by ShaunGamer
Usually because of the way walls are abstracted into the game.Units beside a wall can usually fire over it. When they do so, the targets are in line of sight and their targets have line of sight to them. Because of that, it is just easier to say a wall doesn't block line of sight during combat. However it does make it harder to hit units behind a wall, which is why there is a hindrance value.
Some Games include "high" walls that do block line of sight, but it does so in both directions and no combat can occur across that hex side.
Other games may include more detailed rules that enable you to use walls to block line of sight by "crawling" or "stealth movement", but V&V (like a lot of other war games) does not.
I hope this helps!