Because indirect materials are not critical to the corporation and savings are typically unexpected surprises. Ariba is an example of a company that is based on this assumption. The problem with this assumption is that corporations place most of their focus and resources in activities involving direct material purchases, but direct material purchases are managed by legacy systems that are inherently difficult to upgrade. Direct material purchases involve contractual relationships. Indirect material purchases are based on casual relationships.
Big companies buy lots of simple stuff.
IBM buys a lot of pens, GM buys a lot of name tags, and Sears buys a lot of light bulbs. Hence, the assumption goes, saving IBM one cent on every pen they can save IBM millions of dollars a year. In tough times, though, corporations have many ways available to extract savings in these indirect materials including, most importantly, simply buying less. However, every penny IBM saves on components that go into its computers, every penny GM saves on steel that goes into its cars, and every penny Sears saves on products it resells to its customers is a saving that cannot be acquired by simply buying less. You can provide significant value to a corporation if you save costs associated with critical purchases that cannot be postponed.