So, let's first get clear about what "oil" means. In common language there are two "oils" contained in food: "oil" and "essential oils".
"Oil" means the common, cooking liquid fat, that has a more neutral taste and is used for various applications (deep-frying, stir frying...). Main composition of this is unsaturated fats, and the oil comes from the so-called oleiferous plants, which are plants that use oil as nutrition storage for their sprouts - e.g. peanuts, sunflower, canola, olive, flax seed, soy...
"Essential oils" have a very different composition and application - they contain a very diverse mix of components that give spices, herbs, flowers and other seasonings their caracteristic aroma (or "essence"). These components are usually volatile and present in very small quantities in the natural product, and of course, since they're present in very small quantities in the seed / herb, you need a lot of it to make a portion of essential oil, therefore, those are really expensive.
One of the methods for extracting this oil is to steep the aromatic (spice / herb / flower) in common oil or fat - then you get aromatized oil in a quantity you can handle with more ease.
When you hear that cumin seeds and turmeric (which is a root, not a seed) contain oil, we're talking about the essential oil. The oil is there, in a very small quantity, that's the reason why you cannot see it with the naked eye, unless you grind large quantities of the seed very finely to directly obtain the oil. You're also right on oxidation and evaporation - this is why pre-ground spice loses its punch very quicker than the whole thing.