Because some people have either been hurt too many times, they've seen others get hurt too many times, or they just don't see the benefit of having a long(life)-time significant other. I love my wife (and kids) to absolute death, but there are times that I see things I could have done differently if I was single. I'd never consider giving my family up now that I have it, but I can understand why some people choose not to go my route.
Thank you very helping