We design and build various structures using concrete because we need its strength. Pouring concrete allows for it to take virtually any shape we need it to and its strength is in its solid form. The weak links in other materials like the mortared joints in masonry and fasteners like nails and screws in wooden and metal structures are not needed. Concrete structures are indeed solid literally. But these structures are affected by the same outside forces that affect all other types of structures. Since concrete is ridged, it lacks flexibility and therefore is more prone to crack or break in response to outside forces like soil settlement as well as interior issues such as rebar corrosion.
Epoxy injection is a very economical option to repair these non-moving cracks (not control or expansion