First Answer: Lack of practical knowledge in life and wisdom.
Second Answer: Some may know about the impact to the kid, yet they chose not to for their own convenience.
Third Answer: It can not be blamed entirely to an environment with broken family but it could be one factor why it happened to those who had been exposed.
Forth Answer: Again, genetics could be another factor based on one's temperament, but not entirely is the main reason why one is likely to end up with a broken family.
The narrative that life will get better once you get separated is an over-opportunistic assumption and naive, which many had bought in. You could blame it on how relax women are today about s3x because of birth controls which could result to unwanted pr3gnancies and men pursuing pleasures rather than the value of becoming a decent husband and father. One may say that a legal abortion could avoid that kind of situation but that is a slippery slope to agree or disagree. If we at least adhere to the religious idea of the sanctity of marriage inline with truth and humilty, what else is better than that? It is quite sad that we get used to convenience due to the technological evolution that we somehow surrender to it rather than pursuing what is ideal and meaningful.