Good question!! There are all kinds of reasons. Usually, concrete slabs around your house tilt or sink because they were placed on fill dirt that was not compacted. This can happen just about anywhere.
For example, what about a sidewalk that is next to your foundation? How about a garage slab? What about a portion of a driveway or sidewalk that crosses a utility trench (one that contains a water, sewer, electric or phone line?) These situations almost always involve an excavation where dirt may have been simply dumped in after the work was complete.
Herein lies the problem. Buildings are built on dirt all the time. If done correctly, a house built on dirt will not settle or develop structural problems. The trick is to build the house on undisturbed dirt or soil that has good strength, or build it on compacted dirt that will support the structure.