In recent times, most of the people I date (or just talk to) are experienced: being divorced, inconclusively separated, or otherwise similar situations.
Just this week I was talking to a friend, who asked why their partner "gave up", and I referred them to another friend who "gave up" on their partner. Note this concept of "giving up" is completely legal and coherent with the regulated status of marriage ... but some people will think, it is not coherent. The reason is that, many such people are thinking about other extra-judicial concepts related to the word, "marriage", such as mutual promises between individuals, or other religious ontologies.
Upon further conversation, I discovered that my first friend simply accepted their divorce without any pursuit of the legal protections regarding the division of assets between the couple. So I concluded, my first friend did not have to get legally married as they did not care about the regulated protections of marriage - they could have instead focused on the extra-judicial ceremonies and development of the marriage, with any resources saved from the avoidance of a legal marriage.
As for my own answer to the initial question, I think many people simply don't brain what they are signing up for, when they execute a marriage. Then later they realise their mistake, and act upon it.