Many do today, but in those days nearly everyone did.
These days we realize that two unhappily-married parents can often be worse for the children than if they get divorced and continue to be active parents.
A woman I dated about 15 years ago came from a household where her parents clearly did not like each other.
She treated our relationship as being adversarial. She felt like she had to "win" every conversation, and one time even admitted to choosing to take a side that she knew was factually wrong just to stir things up. I think she truly believed that love was constant bickering.
Within a year of her and her sister's moving out, her parents divorced. They stuck together "for the kids", and as a result, the kids got a terrible impression of what a loving relationship is supposed to look like.
These days we realize that two unhappily-married parents can often be worse for the children than if they get divorced and continue to be active parents.