Invariably, these calls have been less than 1 minute and boil down to sorry, you're not hired.
Um... why not just say that by email? Or (for those who insist email is too cold) call me ASAP after the hiring decision, and leave a voicemail if I don't answer?
Why make me wait several days for a <1 min phone call?
Recruiters wait because candidates often don't accept an offer, or take another offer, or negotiations break down after the offer. A good recruiter will keep a candidate who wasn't the first choice so they can act quickly if the job doesn't actually get filled.
I've worked with recruiters and sat on the hiring end. Sometimes when a candidate doesn't get an offer they call the hiring manager to ask why. A candidate with a professional attitude will go through the recruiter, but people often take "you're not hired" personally and demand answers, and at that point they've met the hiring manager and know whom to contact. The employer may want to keep their options open because a decision to make an offer doesn't always lead to filling the job.
It may also come down to priorities. Recruiters don't make any money from candidates they didn't place, so following up won't rise to the top of their daily list. How quickly and honestly a recruiter follows up says something about their professionalism, but also about the likelihood of placing the candidate in another position.