I think it has to do with the mouthfeel at room temperature. The more saturated a fat is, the more solid it tends to be at cake eating temps. The more liquid-y oil just seems moister, more water-like in the mouth. That, plus recipes with fruits/vegetables tend use oil, and they are a little more moist anyway, because their sugars (and maybe fiber, too?) are so good at attracting and holding on to water.
Use the bread analogy- room temp bread with butter has a solid spread on it and while flavorful, can easily feel dry in the mouth if the bread is on the dry side. Dipping the bread in liquid olive oil does a better job of counteracting any dryness.