Desired inner monologue I think they hope to generate in us: "I've already spent 3 calls and a couple of hours talking to <recruiter>, so I might as well continue on with the process lest I end up wasting all that time."
disclosure: I'm a dev and not a recruiter, so take this with a grain of salt.
Example 1: they need to get a rough idea of whether you're a jerk, whether you can communicate in the required language, whether your resume is BS, etc. Remember, nobody's paying the recruiter to find you a job - somebody's paying them to find high-quality candidates. Their job is to sell you, so naturally they want to know what they're selling.
Example 2: simple context switching. If your job required you to be involved in dozens or hundreds of simultaneous ongoing negotiations, wouldn't you find it more efficient to block out 30-minute calls with each stakeholder, rather than jumping around between a hundred different email threads full of single-question emails?
One could probably come up with more straightforward reasons. As a general rule though, nobody is ever consciously trying to waste your time. They're trying not to waste their own.
There is also a legal concern - phone calls often times cannot be recorded so one can be more loose with their language.
Right now recruiting is Yet Another Dysfunctional Industry with non-optimal behavior that favors job creation and useless busy-work instead of finding optimal solutions.
But if you just pay an agent to run around selling you, there's no longer any value being added. Why would companies listen to a recruiter that just refers any old candidate that pays them?
So I definitely think phones are used because it allows the rules to be bent in a multitude of ways with a lower risk of consequence.
Plus it’s easier to pressure people when you’re on the phone than it is when they’re emailing.
In almost 100% of the cases, if a Recruiter is able to get someone a job, it's immensely beneficial to the new employee, far more so than for the recruiter.
Up-thread they said they explicitly asked for email instead after taking the cold-call. What's the good faith reason for insisting on a warm phone call when it's not wanted?
When you know the name and fact of the person you are working with, you're much more likely to have it stand out in your thoughts as something material.
Sometimes that might feel a bit odd with our view of efficiency and rightly so by some measures ... but it's a bit cynical to wrap everything up in a 'sales tactic'.
If recruiters didn't show value I don't think they would be used so they're not just 'selling' something, they are ostensibly 'providing' something.
It's been a while since I've worked on that front, but I do remember job hunting as a lot of 'empty time' I wonder if it's worth contemplating those direct connections as being material to the mechanics of the system overall.
If not, then getting you on the phone is an absolute necessity along the path to you accepting an offer. It's also practical for covering items like salary expectations, how serious you are about job hunting, outlining the role, etc.
Nevertheless, every step prior to discussing salary and interview with the actual employer/team can be done over email. Recruiters are typically less informed on the tech side anyway so there's really little reason to phone call type interviews/discussions.
I think I did this in reverse with a recruiter when I turned down a job with JP Morgan. He didn't like my reason for turning the job down and kept trying to give me a proper answer and kept calling several times as well as others in the firm calling.
however, not an unknown tactic!