The reason people eat so many simple carbs is because they are simple. They digest quickly and have nothing to slow the absorption rate. This leaves you to feel less full off of more. Complex carbs take more time and thus make you feel more full off of less.
I think you're confusing "simple carbs" with "calorically dense carbs". Simple carbohydrates are monosaccharides and disaccharides, commonly known as sugars like fructose, lactose, sucrose, glucose, galactose, and so forth. Complex carbohydrates are long chains made of monosaccharides, and are commonly known as starches. Many complex carbohydrates that we consume are readily broken down into simple carbohydrates in the digestive tract and absorbed just as readily as bioavailable simple carbohydrates.
Certain (nontoxic) long-chain carbohydrates are indigestible and are known as dietary fiber. Among other effects, these tend to slow down the absorption rate and act as "filler" material that stimulates the stretch reflex of the gut, making you feel fuller. They tend to make foods less calorically dense, since you need more volume to get the same amount bioävailable carbohydrates.
Complexity and caloric density are two different ways of categorization. For example, foods like breads, pastas, rice, and potatoes are calorically dense, but are all complex carbohydrates.
Hope this clears some stuff up.
