I get that some foundations do have sinking issues, but how does having a foundation help? If the foundation is poured and solidifies, what stops the weight of the house from immediately sinking/tilting the foundation?
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For the same reason that hitting a nail with a hammer will put a hole into wood, but hitting the wood directly with the hammer with the same amount of force won’t do anything. The force is concentrated into a much smaller area, which creates a much higher pressure.
Pressure = Force / Area
Say the tip of a nail 300 times smaller than the face of the hammer. Holding force constant (by swinging the hammer with the same force), decreasing the contact area by 300 times results in an increase in pressure by 300 times.
House foundations work the same way. Most of the load of the house is concentrated into the load-bearing walls (like the nails). By supporting those concentrated loads with a stiff, large foundation (like the hammer), the load gets spread out so that the area of the foundation that makes contact with the ground (like the wood) becomes much larger, in turn drastically decreasing the pressure on the ground.
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