In process improvement projects, why should we focus our efforts on those processes that consume the most resources (e. g., labor, space, capital, etc.) ?

Answer: Because this is where we get the highest returns from.

Explanation:

They are the force behind production and efficiency, getting them improved or better means improvement in production or efficiency which literally means there is much reward for time, money, resources etc spent. Their inefficiency could mean doom for any business or project. It could literally lead to loss.