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It’s common because they can usually get away with it. Once the building is complete you can’t tell whether components were missing when the concrete was poured, and not many buildings get hit by aircraft or whatever.
But yes, it does happen, eg. the Ronan Point collapse in 1968:
– Further construction defects had led to the whole weight supported by each wall panel being supported by the panel beneath by two steel rods, instead of being spread evenly along the panel, leading to extremely high stresses that the concrete was not designed to withstand.
– The strengthening brackets which had been fitted during the rebuilding were in many cases not properly attached, since they were fastened to hollow-core slabs, and in many cases they had been bolted only to the thin concrete surrounding the cores, which was inadequate to take the stress.
– The concern […] eventually led the council to evacuate the building, and then to demolish it in 1986 in a forensic manner (rather than, for example, using explosives). When this was done, the extent of the defects found shocked even some of the activists, such as the architect Sam Webb, who had been lobbying for years that the building was unsafe.