Not even close. People can be friends with benefits because they don't have time to put a lot of effort into a real relationship. They might also just be interested in having fun and gaining sexual experience. They don't have to be hurt or afraid of commitment. That might apply to some, but not all. A lot of people just aren't ready or don't have the time, so they'd rather have flings to satisfy their sexual needs.
For starters, I couldn't find someone I was genuinely attracted to as a FB. I've been hurt so I felt compelled to tell these girls the truth from the beginning and cut it off slowly with them if they started to seem emotionally invested. So I just stopped looking for anything other than a real intimate relationship, and eventually stopped looking for that because its like being struck by lighting in the same spot a year apart.
Not necessarily, I do think that sometimes having someone that you can rely on and want to sleep with doesn't mean you have to date them. Some people genuinely don't want to date because of various reason and then yes there are those who are afraid to get hurt because it's not an easy thing to handle, getting hurt can definitely change the person that you are. BUT also I don't really believe in "fwb" because I don't sleep with my friends, I just call them someone I'm sleeping with.
I think there is a lot of friends with benefits when people are younger because not too many say 18-24 year olds are trying to settle down right then and there and start marriages and families and long term relationships when they are just young and having fun. Some times people need to go through these phases to be able to understand why a relationship may be better for them. I don't think it's always hurting or comittment issues.