The most likely culprits are journalists back when they wrote things by hand. The final comma took extra time to write and thus was omitted. Somehow that trend has bled into popular thought and prevailed as a poor stylistic habit. In fact, A Manual of Writers, Chicago Manual of Style, Gregg Reference Manual, Scientific Style and Format, and my favorite Elements of Style all state that the serial comma should be used in a series of three or more terms with a single conjunction. Who argues against it? The Associated Press Stylebook. Unless you work for the AP, use the bloody serial comma.
Now for one of my favorite examples, which I once saw on a magazine cover: Rachel Ray finds inspiration in cooking her family and her dog. Strangely, no charges have been brought against her.