Why don't guys prefer to have an official relationship with girls whom they had a friends with benefits relationship or a hookup with?

So I noticed that many guys are happy to have friends with benefits relationship or hookups with girls but they don't consider these girls as girlfriend or wife material. A guy I know used to sleep with one of my friends for around one and a half years and he later said that she's not his type , which surprised me, because he was literally calling her for sex everytime and they uses to go on dates in clubs, dances, smoke together in his room, even lived together and even introduced her to some of his friends as his girlfriend but to others he never considered her as his girlfriend and he never addressed her as such. He just says the term "friends" to describe her.

Why don't guys prefer to have an official relationship with girls whom they had a friends with benefits relationship or a hookup with?
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