Short answer: They're slaves to their genes.
Long version
Because their denial is so strong they can't admit their own mistake, and only feel better by forcing the same bad decision on others.
The majority of people are unfit economically, intellectually or emotionally to raise children, yet they will continue to do so, creating more unfit parents for the next generation. Out of the dozen friends I have that have had children, only two are actually good parents.
EDIT: I'll take the position seriously when the parent of a teenager tells me. Until then, pets are so much better.
Long version
Because their denial is so strong they can't admit their own mistake, and only feel better by forcing the same bad decision on others.
The majority of people are unfit economically, intellectually or emotionally to raise children, yet they will continue to do so, creating more unfit parents for the next generation. Out of the dozen friends I have that have had children, only two are actually good parents.
EDIT: I'll take the position seriously when the parent of a teenager tells me. Until then, pets are so much better.