Because they are external from the hiring company, competing with every other 3rd party recruiter out there, as well as the direct hiring of the source company. If they gave you too much info, you could bypass them and just apply directly to the job.
That is the same reason that one open role results in dozens of similar postings for it - different recruiters all posting the same gig.
That is the same reason that one open role results in dozens of similar postings for it - different recruiters all posting the same gig.