It's weird to imagine devious hidden motives like this, when recruiters already have obvious natural reasons to want to do a phone call.
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.
Another element to a phone call is commitment. If you are casually looking and only entertaining emails then you're probably not an active candidate and are a waste of their time. I don't even respond to recruiter emails anymore since I assume they are email bots.
Side discussion: Why is it that they have to find someone for the company? Why can't I instead have an agent that goes around and sells me to potential companies?
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.
The value that recruiters offer is delivering good candidates. If you convince a recruiter that you're a good candidate, then they will run around selling you to companies! And companies will listen, because the recruiter has build a reputation by delivering good candidates in the past - that's how recruiting works.
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?
I mean you could if you wanted. But why would you want to pay someone to line up interviews for you? Agents don’t get you jobs. They get you visibility.
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.