1. They use much larger and higher quality image sensors, which gives the best picture quality.
2. They can shoot in RAW format which allows you to touch up your photo however you want without the horrid jpeg compression.
3. You can shoot photos in much faster succession.
4. You can change the lenses for a wide variety of purposes.
There are many reasons a lens can be expensive. You have to realize that its more important than the body it when it comes to sharp, high quality photos. Also you will have them for a very long time. You will maybe replace your camera body 4 or 5 times before you get rid of a lens. What makes them most expensive are the following:
1. The range of the zoom, very wide angle (14mm) to high telephoto (up to 800mm). A lens that covers a wide range can cost a lot since it eliminates the need for multiple lenses.
2. The aperture. The very expensive ones you're finding all have f/2.8 don't they? They are fast professional lenses. It means that the camera is capable of shooting at the same high aperture across its entire zoom range, instead of consumer lenses which might do f/3.5 at widest and f/5.6 as longest.
3. Special purpose. Some lenses are made to excel at a specific role, and they can be costly. A macro lens is expensive, but you can hold it an inch away from your subject and focus clearly on it.
Your L100 will not take anywhere near as good of photos as the XS. They will all be low-quality and look very flat. By being able to adjust aperture and shoot in raw format, you can give your photos dimensions that you would never be able to without a DSLR. Seriously buy one and take some photos, you will not regret it.
Your next question on here will be, "Good zoom lens for Canon XS?" lol