Why was the trench warfare so costly in humans?

There are many reasons as to why trench warfare was so costly in humans. One reason was because men were ussually in the same place and as a result, the enemy knew where to strike. For example, the enemy would know the exact location of a group of people. The enemy calculates the possition and fires a mortar at that direction, killing a bunch of people. There are many other scenarios like this one ... aircrafts, bombs, ect.