The slab part of the foundation generally is meant only to transfer the load of the house onto these footings rather than support the weight itself, sort of like a desk transfers the weight of it's contents onto the legs.

While this can help resist movement, it all depends on the drainage and expansion qualities of the soil where the bottom of the footer rests. If the area sees periods of extreme drought in highly expansive soil and the drought 'reaches' the bottom of the footer then you'll end up with significant movement as the weight of the foundation and home settles down into the void created by the now dry soil, while the opposite is true during periods of extremely heavy rainfall.

When you combine periods of extreme drought and heavy rainfall in close succession of each other on highly expansive soils, pretty much any slab that is not supported by bedrock in some way will be at risk of cracking due to the frequent seasonal movement of the surrounding soil.

This can be mitigated somewhat during periods of drought by 'watering' the areas surrounding the foundation, and during periods of heavy rainfall by having a properly graded home site that routes water away from the home in every direction. Unfortunately these are mitigations that require monitoring by homeowners who may not be aware of these issues at all.

Ideally prior to building any permanent foundation a core sample would be obtained from the site and analyzed to determine the footer depth necessary to compensate for the 'worst case' rainfall and temperature fluctuation in that region, additionally accounting for local movement due to topography.

Except most folks building out in my area are just trying to make as much money with as little investment as possible, and thus do just enough to not be liable if any of the above scenarios conspire to create problems for their long-since-forgotten customers.