What's the big deal with referring?

So here's what I don't understand. People who have jobs that are hesitant to reach out and offer to refer a job seeker to their place of work. What's the big deal? It's not like one is asking them to
"get" them a job. You're just trying to get your resume noticed. In a world where HR people are getting slammed with resumes, all it does is say "Hey take a look at this person's qualifications. Perhaps you'd like to talk to them"

That's all. And with the size of referral bonuses at some companies, perhaps it'd be worth their while?