There were reasons for the way the fighting was conducted too, and the popular idea of "trench warfare" being the norm on the Western Front for the whole 1914-1918 stretch is misleading. As with pretty much everything in life, things were a little more complicated than that.

The excellent /r/askhistorians forum has had some good coverage of this area in the past. Try this overview for a start:

https://www.reddit.com/r/AskHistorians/comments/37epxy/i_kep...

Or this seven-part epic answer regarding Verdun and the Somme more specifically:

https://www.reddit.com/r/AskHistorians/comments/32t8bn/world...

or more from the Western Front section of their FAQ:

https://www.reddit.com/r/AskHistorians/wiki/wwi#wiki_western...