Some people spend years in an unhappy marriage because they are terrified they won't be capable of finding a new partner. For these people, the sooner they abandon their loveless relationship, the better off they will be. Letting fear hold them back only makes the problem worse. They will get very comfortable as an unhappy person in a bad relationship. This will lead them to expect all relationships to make them unhappy if they live this way long enough.
Getting a divorce is easy enough these days. It is mostly a matter of equitable property settlement and paperwork. The courts are generally willing to accept mediated settlements as long as both partners agree. Once this is done, both people are free to pursue their own lives and find new relationships. For both of them, this often means learning to date again. They are seeking a way to find the happiness they felt at the beginning of their marriage.
Some people wait before they begin dating again. They are the types of people that want to protect themselves from getting involved in another bad marriage or relationship. They also recognize they do have a physical need for intimacy. These are the people most likely to find a fuck buddy. For the newly divorced, this gives them a chance to explore their options without a commitment. Fuck buddies are often in the same place in life, so they don't want to bother with commitment either.
Society has frowned upon casual sex relationships. These are seen as being foolhardy and reckless. People have come to see that jumping into a bad relationship or marriage is just as reckless and can ruin their lives. They would rather be with someone for physical intimacy so they can wait and find a good match for a real relationship in the future.