If you make a habit of reading the labels on bottles of a store-bought salad dressing—including the fancy ones—you are unlikely to find olive oil on most ingredient lists. This is partly a matter of cost, but it’s also a matter of taste.
Olive oil, especially good olive oil, has a pretty assertive flavor. This makes it great for bread dipping or drizzling over roasted vegetables, but it doesn’t make it a fantastic salad dressing base, particularly if you’re trying to highlight another flavor. Olive oil just shouts over the other ingredients, muting other tastes and muddying the profile. (For example, the dressing for this breakfast slaw was coming out terribly until I changed the oil from olive to grape seed; olive and blood orange did not get along!)
Do not misunderstand me: It can be great in a simple vinaigrette with lemons and garlic, but it is not a one-bottle-fits-all solution to salad dressing. A neutral-tasting oil—such as grape seed, canola, or plain ol’ vegetable—will serve you much better (though grape seed is my favorite). These blander oils are unexciting on their own, but they do what olive cannot, and let other ingredients shine. Beyond using them as a base, you can play around with ratios, using your neutral oil to cut more flavorful oils if you want just a hint of their flavor in your vinaigrette. They can also help liquefy other fats that are solid at room temperature, such as bacon grease. Grape seed oil may not sound as sexy as olive, but sometimes being bland and boring is what you need.