Cells are not perfectionists. When DNA is damaged, cells repair sufficiently to keep their machinery working. Such workmanship is also found in wounds, where tissues are mended well enough to recover functionality, but not blemish free.
Geneticists Serena Nik-Zainal & Benjamin Hall: “Damage to DNA occurs continuously. DNA quality control mechanisms, such as DNA repair and replicative pathways, mitigate that damage and preserve the DNA sequence. Considerable somatic mutagenesis is common, the inevitable outcome of cellular wear and tear. Pathways may be naturally limited in activity, resulting in permissiveness to mutagenesis. This is a prioritization of survival over genomic perfection, given that most DNA damage is inconsequential and thus, affordable.”
Reference:
Serena Nik-Zainal & Benjamin Hall, “Cellular survival over genomic perfection,” Science (15 November 2019).