Because people will carry a certain number of recessive genes, and when children are born between close relatives, the recessive genes will overlap highly, so two recessive genes will combine into one dominant gene, which will lead to children's illness. And it will cause atavism, and few children are healthy.
In 2008, a shocking case occurred in an Austrian city called AmStetten. In the local area, a 73-year-old animal father imprisoned and sexually assaulted his own daughter for 24 years, during which he forced her to give birth to seven children. In the past 24 years, Elizabeth has given birth to seven children because of constant pregnancy. They are all locked together in the basement.