Many reasons! My best guess is they want to keep the job confidential/exclusive (especially if they're from an agency). If the candidate is not interested, why bother sharing the job with them and have them share around within the industry so a competing agency can pick the role up and send their own candidates to the clients. For candidates. it may seem to be a job application process, to an agency recruiter - every job/candidate is business.

When I was a Recruiter in my agency days, I do not share the company/role upfront before I know for sure the candidate is looking for a, say Production Manager in the Automotive/Robotics industry because candidates tend to speculate. Sometimes, these roles are not advertised and are given to me exclusively by my client so there's abit of kiasuness there. So yeah, how I'd do it is - speak to candidates to gauge interest first, and share if only they're open to know more.